Only two I know of have FM as a hard requirement (whether or not your school offers FM: warwick, cambridge), a few of the other top ones expect you to have it if your school offered it. You'll have plenty of choice without FM, but if you want access to slightly more universities you might want to research doing an AS in further maths. (some unis accept this for a reduced offer)
a) You don't need to worry about getting offers from many maths courses. If you have the right predicted grades, you will get an offer from most universities you apply to. (possible exceptions being oxbridge, imperial, durham, st andrews off the top of my head)
b) If a university does not require FM it won't assume FM knowledge and will cover the content from scratch. Even those who do will re-cover some A-level content in a more brief and university-style way assuming some existing familiarity. For example, you might be given an integration/differentiation identity and a proof (technically covering it from scratch) but you won't have loads of examples accompanying it similar to A-level, they'll assume you basically know how to use them and move on. (not sure if I'm being clear here so lmk) They will therefore accommodate to some extent people who didn't do FM. You might find that quite a few people will have FM regardless, so some prior exposure (say reading of important topics over the summer) will allow you to hit the ground running.