The most practical method of self-defense is keeping yourself out of trouble. If that's going to be difficult, since you're going to be living in England and there aren't any guns to be worrying about, you can do what all of my friends that live in DC do, run. It's always worked for them whenever someone wanted to mug them (they are pretty fast though).
If you're really are dead set on learning a martial art, my advice would be Krav Maga as mentioned above. The Israelis live in one of the most hostile places in the world today and most of their techniques for hurting people work. You could also try out any other run of the mill self-defense class since these will probably be very similar to a civilian Krav Maga class (they're not going to teach you killing techniques unless you're in the army).
How ya doing, buddy?
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