You don't need to have studied a foreign language while in school to do linguistics. It's arguably of somewhat marginal benefit, unless you want to specialise in the linguistics of that language. Linguistics is the study of language, not languages - i.e. language in the abstract. It is in fact often quite abstract in some areas (semantic/syntactic theory for example).
So having a language background isn't necessarily that useful (also outside of ancient languages i.e. Greek and Latin you don't really do that much formal grammatical training in most MFL subjects I think, rather just learn as much as you need and kind of wing it and then learn the grammar properly in an MFL degree if you continue to it, I gather).
In theory any mix of subjects is fine (and in fact specifically taking a mix of subjects might be good preparation, as it often includes a wide range of approaches that can range from more humanistic or social scientific to more natural scientific or formal scientific). Those with more experimental and/or phonetics content may want or require a science subject (e.g. UCL's experimental linguistics route), although most do not.
I'd say just take what you will get the best results in. The only really directly related A-level is English language, which includes some linguistics content. However a lot of that A-level is still unrelated so if you don't really like creative writing, I'd say not to bother. If you want to apply to UCL experimental linguistics or PPL at Oxford then a science A-level (e.g. biology, physics, chemistry, maths) may be necessary.