Great! Let's try to precise things a little bit so the answers can really help you out:
Your profile says you are a student, 18 years old with interest in swimming & MA.
I will assume that you are reasonably fit and able and have some time available. At present you have no affiliation, and did not state a kyu/Dan grade - What is your level in MA?
Could you possibly let us know what attracts you in MA?
It could be for competition, getting fit, gaining self confidence, because it looks really cool, etc... or any other reason or combination of reasons.
From there, I am sure forum members will come up with good stuff for you.
IMHO, if you are a complete beginner in any of the styles you mentioned, choosing a dojo with a top notch instructor and a good environment, atmosphere, etc... is more important than any other consideration. You are darn lucky, London might well be the location in the world with the better choices...
I read Gary recommending a dojo and an instructor for you --- If the location is convenient and you like the place, you can safely trust Gary & go for it.