Not an easy one. This is a long post, but I'd like to make a few points and let you know of my experience for comparison:
On the one hand i do agree with the others that say FY1 is about learning organisational structures and when/how to escalate concerns. You refer to being behind other FY2s in terms of knowledge - FY years teach you minimal knowledge - in fact what you forget whilst filling out the endless paperwork almost certainly greatly outweighs anything you learn - but you do get some 'real world' knowledge in terms of how to deal with uncertainty, when to seek help etc. And that's important.
On the other hand, you (like many others) want to get to specialising ASAP and i think its pretty universally agreed that your FY years are the worst years of your career, largely due to rotating into jobs you have no interest in, and being the bottom of the ladder when all the **** lands. I personally would really be thinking twice about volunteering for an extra year of it if i had the choice. In fact, i would go as far as to say i would choose not to do it, even without an internship year! Fact is, i dealt with far (far) more on my elective and didn't mind the responsibility then, so i could probably have dealt with it afterwards too. But then i was familiar with the UK structure, so maybe that's why.
Also to consider: plenty of EU grads do go straight to FY2 and most are fine. Its hard and you have to adapt, but its fine. I worked alongside a Greek graduate in my first FY2 job, and no concerns whatsoever. As a UK national you will even have some cultural advantage on them, so should be even less risky for you
There is also the big financial knock you would take by doing an FY1 year - you'd be 'paying' circa £10,000 for the privilege vs going straight to FY2.
Below I've briefly summarised my FY1 and FY2 posts so you can see the difference you could expect:
I think its up to you to decide whether you could shoulder the increased responsibility straight away and deal with an especially horrible first month or so, or want an FY1 year first. However, do not overestimate what FY1 year actually teaches you and there are significant downsides by choosing to do one.
Hope that helps.