Chocs, are you implying that it's not possible to work full time and do well in further or higher education? Thousands of people every year work full time in professional roles whilst still managing to achieve merits and distinctions at postgraduate level, or firsts and upper seconds at undergrad whilst studying on the side. Not easy, of course, but it's not exactly uncommon.
Everyone who makes it through to the final stages of something like the FPP will have done extremely well at the aptitude tests, exams and interviews during the previous rounds, on top of a strong educational background. It's not like anyone's getting through just with good A levels alone, so why wouldn't they look for aptitude and qualifications, not to mention the whole list of other attributes they'll be ticking boxes for on their clipboard sheets. Considering the number of really high quality candidates at the final couple of stages, they're going to be getting it broadly right. Is there a workable system (as far as application numbers go) if they lower the qualifications?
Having an advanced maths, physics or science background will obviously help with some aspects of the training/job but whatever the makeup of your subjects at A level, surely it's more about showing a capacity for learning and managing workload? G-FORC3 is being pretty sensible.
Of course, everyone will always defend their own perspective especially if they, probably quite correctly, think they have the right skills and aptitude. There are different rules for different routes, all you can do is push your efforts towards whichever route you can jump through the requisite hoops for at the time. Some routes are blocked by qualifications, others by finances. Frustrating? Almost certainly!