Bubbles as ever is right. I dont think anyone actually requires FM for anything, however on some courses it would be rare not to have it, and they might then be seeking assurance that an applicant was capable of achieving a top grade in FM if they had had the opportunity of taking it.
But these are for the very mathematical courses, including UCL. Bubbles is absolutely right. Places like Bristol will not require as much maths on entry and their degrees will be the launching pad for good careers.
To some extent it depends on which doors you want kept open. The advantage of UCL/LSE/Warwick/Cambridge is that they open the doors to some of the more technical/mathematical jobs as well as the more general ones. But if you are not interested in a career using quantitative techniques it is competely irrelevent. And maths/engineering can get you to the same place.
The entry requirements for the Masters my son took are "First class honours degree or equivalent with strong technical background in engineering, physical science, econometrics, statistics, mathematics and/or economics." A least one of DS' fellow students was from the UCL economis degree.
But assuming OPs son is not interested in that side of things, I dont think it matters. Though a good tip up thread is to consider international recognition. A lot of recruiters in international consultancy firms will not be educated in the UK. It does no harm to have been to a University with an international reputation. Especially if you also pursue UCL's brilliant study abroad options.
And in terms of studying in London, it is horses for courses. Hugely international, very focussed, guest speakers from everywhere, and with a different vibe. DS had a great time, and DD is hoping to spend her intercalation year there. But not for everyone.