Much props for making this thread, that alone is a very big step - and your posts show you clearly have a good head on you so if all else fails you'll have that to guide you.
It's an incredibly tough thing this because you can't know which way your life's going to go, and naturally you'll worry that you may be filled with regret in doing either the dropping/not-dropping. I've read the thread, and I've been in a position where I spent the year direction-less and unmotivated, and am now at a slightly better (but not at exactly where I wish to be).
I spent the past year seriously unmotivated because a lot of my subject areas didn't interest me the slightest - I ended up missing a lot of classes and subsequently failed pretty hard. I spent a lot of that time down - I slept in a lot, filled my time with doing meaningless things during the nights, and truly had very little social interaction during that year. The social bit seems to apply to you less, but bear in mind I was in my final year of college/high school where there's a lot less people. I think the uncertainty itself was one of the reason why my workload and stuff not done just compounded - how can you commit to something when you're not truly sure if you want to do it? How can you put the hours in? Truth is, you can't really, it makes no logical sense to be doing something that you yourself don't fully agree with. My story right now is a little happier than before - I did a whole variety of A levels (5 As's) and found one that I thought I could go with - Computer Science, and right now whilst I'm still suffering from the after-effects of doing very little work during the past year, I'm finding it bearable. Though going back to the uncertainty issue - I originally got rejected from my insurance (also a Russel Group) for missing my grade in Maths by 5 marks - that week following on from results day was literally me struggling to cope with the constant depression. I did my utmost best to fight them and get them to let me in - but the uncertainty of my life - I really had nowhere else to be, no plans for a gap and didn't want to settle for clearing and do a course I wouldn't be okay really got me. In the end, life being what it was through me a lifeline and gave me a remark that took the grade up, and that helped somewhat.
But there are a few reasons to why I'm doing better now (not to say I still don't have my bad days, or the first half-term of uni where I had a few weeks of it due to ridiculous workload), and I'll try and relate them to you as best as possible so you may gain something from them below.
But firstly - in my mind the answer to your question, the best one would be to take year off, and then see where that takes you. If you want to come back to the degree, then great, if not then perhaps it really isn't the one for you. But this option, gives you a break, but also gives you the guarantee of having someplace to be if it all goes ballsup. But really the only thing keeping me in uni and doing the work is because I have some interest in it - some small area that I wouldn't mind doing into that I can sort of relate what I'm doing in my degree to. Without that - I really wouldn't be able to cope. I took Maths, Chemistry, Physics with very little interest in them - I seldom ever put the work in. As I said in the previous paragraph, if you don't have a reason to really want to be doing that (aside from the guilt factors like 'you should feel privilleged' and 'the alternative could be worse' etc), then you'll really struggle. In my mind, any time you buy now towards a year off would be time gained. I mean be real - you're not going to wake up in Easter and find suddenly you love your course. It'll just continue.
Anyway on to the aforementioned improvements in my life that may be of use to you:
1) You have to be doing something concrete.
I don't count the degree as 'concrete'. Concrete is something formal, that requires you to do, that has other people holding you accountable to do, but something that achieves something for you. A job is concrete. From your hobby list - none of them themselves are concrete if you do them on your own. However, if you find a music partner (or tutor, but I say partner as you mentioned your finances), a club that you enjoy where you can write your book with, some formal group that you can join, a short course of some sort. Because doing your hobbies on your own assume you'll motivate yourself to do them - and that's one of the toughest things for people to do. For me, my degree isn't concrete either. But what are: 1) I took a short course in web programming. It was weekly for 10 weeks, £400 and from Oct to Decemeber, but it was the only area of programming/computer science (and that's not in my degree hence so I count it as something separate) that actually interested me. I did the set work. I enjoyed it. I learnt stuff. It'd been a year and a half since I'd felt that. I'd recommend that if you have the money or can raise it. Do short courses. 2) I started attending a weekly group that I feel good around. Albeit I knew some of the people that went, but I went because the topic interested me. It was a free sort-of-religious gathering, but more-so where people come in and give short talks on areas of the religion, and whilst learning things I also got to interact with some great people. You never know whom you'll get with groups, but people on balance tend to lean more towards the good end, so you'll more than likely get lucky. (And on a side note, I don't know whether you're religious or not - but I find their comes great willingness to help another out in pretty much all faiths - I lost my faith during my downtime but attending the weekly place of worship has got me really up. If you're in the London area, I'd be more than happy to give you the details, it's not faith specific and mostly I just go for the good people (all our age)).
Those are just two things, but those two things were pretty huge and pretty much outweighed any downers - even though it was only two days a week, it made them good weeks. Aside from those, I did smaller things too - I started playing footy every other week with the uni too, which is a great way to relieve stress, and I go gym occasionally now too which is also great (any exercise in my mind is great for tough times - but again not the do it your bedroom type - concrete in joining a footy group or going gym).
Apologies for the length in hitting home that point - but it serves as a warning to you - don't leave your course without concrete alternatives. Those two that I mentioned above, the course and the group, I had in mind even when I thought I'd have to take a gap. Find concrete things to do BEFORE you leave. Otherwise you'll leave and find yourself looking for things to do rather than actually doing them. Make this your absolute priority above all else if you do decide to take the sabbatical - find concrete things. Go all out and get a job, find courses, go onto meetup.com and find groups and check them out beforehand and find ones that you like - make concrete ways of achieving your goals that are reinforced by others - not just your willpower.
2) You need to become good at something.
A lot of my depression stemmed from being 18 years old and not having one single useful skill - nothing at all. Thankfully the short course has given me a base to build from - I still would only fetch minimum wage if I went to get a job in Web programming, but I'll be following up with another 10-week follow on short course that will hopefully have me okayish at it by the end of the academic year. If the degree still doesn't work for me - I'll just follow that. For you - it could potentially be your hobbies. If you make them concrete, and become okayish at them (also, don't aim for 'brilliance' in my view, it only makes things worse. I've learnt to get rid of that and be at ease with with accepting I may not do brilliantly at the course, but that's okay, as long as I learn a decent amount and enjoy it) - then you'll have those skills to use should you also not decide the degree is for you. Even a little thing, being good at an instrument, it helps. Perhaps you do voluntary work and find you really enjoy that - it doesn't have to be learning something - you're good at that and in the future you could use that as a starting point for a new path. Essentially, don't be in a position where you're a nobody with nothing to offer - learn something and be good at it, or be go out there and do things. Again the religious group helps a little with this too - if all else goes balls up I still have that to give me some inner strength through tougher times - it doesn't have to be a religious thing - something like the voluntary work or helping others or whatever could work.
3) You need to find what you want to do.
This ones tough, and really causes you so many problems through the uncertainty. I sort of found out I'd like to do programming sort of stuff through looking at it over time, doing it as an A level, and now actually doing it. But I was incredibly unsure - when I was applying for uni I was stuck between two choices, it's only with hindsight that I can say perhaps I'm in the right direction. Obviously my path is still fairly conventional - I did do things beforehand that got me to the degree, but I was similar to you in that I didn't know whether I really wanted to do Computer Science.
But I stress, it takes time to figure it out. You need to find something that'll keep you going if you do it. Something you want to do. Through doing concrete actions as above, you'll be more at ease to have assess your life and gain new experiences to help direct you - far better than simply living life as a bundle of confusion.
I don't really have that much practical advice for this one, just that it takes time to work it out. I don't know you or your journey really, what interests you and what doesn't. It was essentially a whole load of things that made me go to where I am now. It's still not perfect, but it fits better than the rest. And that doesn't even mean going to uni - you could find you enjoy a job or something and you'd like to work at progression on that route. You'll get time to work on it - but the more practical you are the better. Aside from the practical things you do from point 1, you could try and fall in love with MechEng. I sort of did this with Computer Science, you could go out and find people who're working now, see what their lives are like. Meetup.com or your uni must be bound to have some relevant links. If it still isn't working for you, then perhaps do the same for other areas you think may work for you. In the end - I think you need to find something that offers you the promise of a life you enjoy. Although that sounds like something seriously hard to achieve, really it's not that bad - it's merely the promise of some dream. For me, either getting enough knowledge to make my own programming stuff and perhaps make some money, or do an entry level job in web programming and work in companies I enjoy with work I enjoy. Find what path will offer you something that sounds reasonable.
Aside from that, I'd say stuff life ignore things that bring you down, like seriously just forget them, but really I stress the importance of the above 3 points with particular focus on point 1. You need to fill your days away from the possibility of just wasting them - and that means doing concrete activities. Do that research before you drop if you do go down that path.
Obviously the reality of leaving uni is a tough one itself, there's so much to do and so many people to talk to before you're officially out, but imo it would be best for you. The fees thing is fine - as someone mentioned you can get loans for up to 4 years - so you could still go back and do another full 3 years. I wrote this all out in one go so feel free to bring up anything that seems a jumble, and feel free to send me a message through pm if you wish at all mate.