This is a question I hear a lot, and something I have personal experience with. The short and simple answer is that it is better to have high UMS in 3 subjects, than to have lots of qualifications. This is for a few reasons:
British Universities put a much stronger emphasis on academic ability, and therefore if you can secure higher UMS scores in your 3 core subjects it’ll put you at a much greater advantage than having lower scores spread out across more subjects.
You often hear about British University admission being much more academic-weighted, but it’s not all about the scores. Perhaps the most important factor which determines a successful candidate, and unfortunately one of the least understood by applicants and teachers in my opinion, is a firm passion and interest in your chosen course. The whole British education system is centred around specialising early, as demonstrated by the limited number of A levels we take (despite the growth of IB here). When applying to top institutions like Oxford, Cambridge and the University of London, they want to see you’re deeply passionate and invested in the course you are choosing. This means that, if you are applying for Economics at Cambridge, they will want you to perform better in and devote more time to your A levels in Economics, Politics and Maths, than in the English A level you’re taking as your fourth option.
Often taking a fourth A level can detriment your scores in the other 3, as well as your application in general. I personally dropped Russian in my final year of secondary school after much discussion with my teachers, because it was consuming a lot of time and I was only predicted A/B-equivalent grade (Pre-U). This opened up more time for me to focus on my core subjects, as well as focusing on extra-curricular activities which demonstrated my invested interest in my chosen course, which significantly strengthened my application.
One final point to consider is that while it doesn’t happen often, Oxford and Cambridge can make an offer of more than 3 A levels. I have a friend who applied to Oxford for English and was given an offer of 4 A*s, so decided to go to the US instead. They cannot, however, make you an offer of more A levels than you are currently taking, so to guard against this it is best to take 3.
All this being said, it does depend on the individual and course. It may be beneficial for someone applying for Natural Sciences at Cambridge to take Chemistry, Physics, Biology, Maths and Further Maths at A level, as it demonstrates a wide range of skills and interest in the course. However it will always be better to achieve higher UMS in 3 than to spread yourself thin.
With regards to modular exams getting phased out in the UK, Cambridge are introducing Aptitude Tests similar to Oxford to replace the AS level data which will no longer be available. Unfortunately, they are only being introduced to this coming group of applicants so I don’t know much about them yet. I’m sure they will still consider CIE A-level percentages though as part of the application, but not in place of the new tests.