Hi! Big congratulations on your place! I am loving RVC and London at the moment
For the workload/transition part I would suggest going over old work (e.g. if you did A levels making sure you can remember a fair bit of it, especially biology) and try and get yourself into a fairly disciplined mindset where so you can keep up. BUT don't stress out too much, they have students from loads of different countries who have gone through different education system- so the first term or two seem to be easing in fairly gently, and making sure everyone has the same basic background knowledge.
There are loads of people I know at RVC in the first year who are your age- lots of Americans who did a pre vet course, and a few Brits too so I don't think you will feel too left out
To be honest I am not too sure about finances if you are over 19 at the start of the course- but if (as I did) you get a student loan, then it should be manageable especially if your parents help you a bit. If not, then there are various bursaries on offer, and you can generally get help if you need it, at least in my experience from talking to other students. I am not going to lie accommodation in London is very expensive, but it is only for 2 years and out in potters bar it is lovely and cheap by comparison. Other than accommodation, I am not finding cost of living much more than usual, though I rarely go out to clubs and I think that nights out are pricier in London. I am very much a country person and was very unsure about London but I have grown to love it and will be quite sad to move out next year!
I hope that answers your questions and reassures you a bit! Feel free to reply or pm if you want.