The only possible limitation is that without FM you might be a weaker applicant for some CS courses (e.g. Warwick, Oxbridge, Imperial, probably Edinburgh, maybe Southampton and UCL as well). Otherwise they don't really care what your non-maths subjects are (except Cardiff, because it requires A-level Computer Science or equivalent rather than A-level Maths, I believe).
I would point out that "Russell Group" is an extremely meaningless metric to be basing your university choices on for UCAS applications. The Russell Group is a postgraduate research consortium (actually it's just a political lobbying group) and this has no bearing on undergraduate teaching (certainly not a positive one anyway...there is a legitimate argument that it might actually make undergrad teaching worse). Further, graduate recruiters do not give a toss whether you went to a "Russell Group" university or not. The only roles which put signifcant emphasis on where you studied is investment banking, and they only care about their internal "target" vs "non-target" universities. Many Russell Group universities are not target universities for investment banks.
For working in the computing sector after a CS degree the single most important thing will be what work experience you've gained and what kind of portfolio of coding projects you have built up to showcase on GitHub or similar. This is entirely dependent on you and has nothing to do with where you study, with the exception that some universities may offer a placement year which would be a very good option.