If it's syntax and semantics that you're interested in, I'd say choose between UCL, Cam and Ox. Edinburgh is good for syntax, but I personally think they focus more on applied ling and language teaching, but the city is GREAT!
UCL focuses particularly on syntax and pragmatics, and they've got famous ppl there... The thing is, you'll probably end up going down their path, rather than developing something new, the influence is just too much (not that this is bad, just something my friend from there once mentioned). Look at what they are doing and if you like it, then I'd recommend UCL (plus it's in London )
Cam has this comparative syntax research group, so that's kinda different and the ling dept. is under modern languages; and at Ox, there is research on the interface between syntax/semantics and phonology, the dept. does philology too - very traditional old school If you are interest in phonetics/phonology. Cam, Ox and UCL all do relevant research.
I think it really depends on specifically what interests you the most, and they are all great unis