A close call, OP. Unlike most above, I lean to Warwick although I don’t really think DS can go wrong. Apologies for the length of my reply:
AI is not replacing CS graduates from top programmes; it is replacing ‘coders’, a word with quite a variable meaning. Even there, a rushed, wholesale replacement is creating some unexpected problems and guess who solves them? Computer scientists.
The COWI institutions have extra cachet in the financial world. Against this, being in London puts you nearer the action. Of how much practical value is this? Warwick Careers Services is excellent and much recruiting is done there. It isn’t difficult to get to London for specific events.
The internships are in London, but these are now ferociously competitive. Only a small minority of students in either degree programme are likely to obtain one. Of course London offers better summer employment generally, but unless you have a 12 mo lease this is no advantage. In any case, it is a small one.
DH is a maths professor in a highly ranked School that sends a portion of students into Finance (usually but not always in London). I am in a Maths adjacent field and we send more than a few. But in both cases, it is much smaller than the proportion who begin studies with the aim of a financial career. This is true throughout the Russell Group.
IMO this is because high school pupils mainly understand Finance to be an opportunity to make money and exert influence. As they gain a fuller understanding of the plusses, minuses and competition, whilst hopefully taking satisfaction from their studies, they may reassess.
One of the things they may learn is that aside from the very top jobs, which are beyond competitive, the salary differential between the tech sector and the financial sector is not as great as they might have been expecting.
I don’t mean the tech sector overall, where salaries are variable. I mean the big tech firms and the tech/analyst jobs at the big consulting firms. IMO these are roughly as competitive as financial jobs and rewards are similar (particularly at the big firms in the tech sector, counting stock options).
The point here is that if DS has a typical experience - and he may not; he may stick with Finance - he may enjoy his studies and may wish to align his career with them. Pragmatically, the ‘CS’ part of ‘Maths and CS’ is likely to dominate in a job search, and Warwick has a better reputation, actually in both fields, than UCL. But UCL’s is also excellent.
From an academic point of view, is there a focus in the Maths and CS programme at UCL, important to DS, that he cannot get at Warwick? Note that CS students at Warwick have the opportunity to take plenty of Maths modules.
One thought: by taking the right Maths modules at Warwick or by going to UCL, with a good degree DS would be eligible for an MSc in Financial Maths. So eg Warwick now, with the requisite Maths modules, deciding what he really wants to do. (He can transfer between the three and four year programmes readily). If Finance it is, what about going for an MSc, possibly in London?
Finally, I always think it is helpful for YP to join the online forum The Student Room. They can ask whatever questions they like of students at particular universities and even in particular degree programmes.
Best wishes to DS.