"but most people say that once they start the proper GCSE work, it is really hard to keep up good grades" -- firstly, this isn't true so don't worry about that. There's not really a big leap in content apart from new subjects you take (like business / CompSci), it follows it's natural progression so you won't go from last Y9 maths lesson being easy to first Y10 lesson being hard - GCSE has a looot of recap and new topics are added in naturally rather than rushed in. Only GCSE's I see having big differences in content from pre-GCSE are History, languages and music.
Basically, you just want to choose GCSE's you enjoy and are good at. The subjects themselves don't matter (not for GCSE, for A-Levels yes somewhat) however you may want to choose ones which compliment your future A-Level aspirations, as well as ones which you just think are interesting. Only your maths, English and science will be looked at for university (and a language if you plan to go to UCL). Of course achieving a strong set of grades overall will help you compete in top universities (more specifically Oxford) and colleges, so that's very important - don't pick up a subject if you have a feeling you wont be good at it.
One tip, don't fall for EBACC - as a student that means nothing for you, and there are no benefits / extra qualifications of doing it. It is purely a way for schools to measure progress, but some staff will try to make it out as if it's going to help you in the future - it wont. But yeah man, there's really not much to say, it sounds like a big decision (and at your age it kind of is) but in the long run it's not really something people will look at. As soon as you earn a qualification above GCSE's (so Level 3 or above) then what you did at GCSE's pretty much becomes irrelevant (again, apart from Maths and English) in terms of getting a job.