Most unis aren't especially fussed about GCSEs, and also most unis consider 8 and 9 grades equally as equivalent to the old A* grade. You still have very good GCSE grades and they won't really be a barrier for anywhere in the UK except for some courses at Oxford or LSE (also some medical schools but as you don't meet the A-level subject requirements for those and aren't intending to pursue that, it's just for reference for others). Probably more of a barrier for highly ranked US colleges (possibly also Canada but less familiar with those) as they consider your overall grade profile through both GCSE and A-levels.
Note that most "top" universities in the UK do not use the UCAS Tariff. They will just set offers based on A-level grades. Also note that even those which do use the UCAS Tariff may not always accept all qualifications offering tariff points. So you shouldn't do something just for the UCAS Tariff points - if it's something you wanted to do otherwise and happens to also have UCAS Tariff points attached then that's fine but don't do it purely for the tariff points. Don't think it's something US colleges are aware of at all.
Equally note that many UK universities are not especially focused on "extracurriculars" in general, and only a few are really particularly interested in so-called "supercurriculars" which are (academic) extracurriculars related to the subject you are applying to. For the vast majority including "top" universities like UCL, you are usually absolutely fine just applying with good grades and having done relevant wider reading around your proposed subject area. Note however US universities do like "generalist" extracurricular activities, although they generally want to see you have done them consistently over a longer period of time (e.g. not just a year) and that you have taken on leadership roles within those and also distinguished yourself both in the activity and as a leader within it.
Note that if your aim is to go into investment banking or management consulting (really the main areas where going to a "prestigious" universities counts, and I'd note even then it's not just what is generally considered prestigious but whether it's one of their target universities or not), your degree subject is not important and you can just as well go into those areas with a degree in e.g. classics, chemistry, anthropology, etc, as with a business/finance related course or economics course.