Only UCL and Imperial and maybe 1 or 2 others require the full FM A-level (Cambridge requires it to AS) but you'd be a complete idiot not to take it. Even if it isn't offered at your school, it's not too difficult to teach yourself, especially with the FM support programme around.
Unis just can't really discern much from a single Maths A-level, considering a whopping 40% get an A in it (the consequences of butchering the Maths A-level to "widen access"). And if you like Maths enough to do it at uni, you should want to do Further Maths.