Unless you are applying for a degree course involving language study, I can't imagine that the majority of courses would require an MFL. Speaking from experience, I applied to Russel Group and N8 universities and I didn't have any GCSEs. I was studying a language at 6th form, but there was no formal grade requirement for it... which presumably there would have been had the universities required a qualification in an MFL. The only thing is that I speak 2 languages already (in addition to the language I was doing at 6th form), but I doubt that affected it really.
I think it is just common sense, generally speaking. If you are applying for Spanish Studies then it would be imperative, even some liberal arts courses or courses like ancient hiostory might prefer it, but if you have good marks anyway even courses like that will probably accept you.