I have a couple of friends who were involved in radio of dubious licencing. To be honest, it's not worth the hassle.
When people think about pirate radio, images are conjured up about how it would be really cool (like that horrible film with Christian Slater, 'Pump up the volume' I think it was called). Where as in actual fact, they're a pain in the ass to do properly.
You really need to live in hi-rise block of flats to propagate a good signal.
No body will ever listen to you because they don't know of your existence, especially if you're having to frequency hop or site hop to stay one step ahead of the DTI.
Your coverage area will also be rather crap, most stations outputs are in the range of kilowatts going into expensive aerials several hundred feet up a purpose built mast. Your's will probably 100 watts feeding into a piece of damp string if you're lucky.
Your output will sound absolutely terrible (there's a reason why proper transmitters cost upwards of £100,000).
When I lived in London, I couldn't listen to most stations at the top of the dial after dark, because some idiot nearby was running a station playing over-modulated Dub Reggae into a badly set up transmitter which bled over everything from 102 meg and upwards.
After you've done all this and set your pirate station up, the DTI (or FCC if you're that-way inclined) will come and confiscate all your gear and probably prosecute you as well. I had a friend at Uni who had been caught running a pirate station and his criminal record (yes it was that severe) has mean't he's had all sorts of problems getting a job in radio and even if he did get a job and promoted up, he could never become station manager.
After all this, don't think I'm trying to put you off. But I guarantee you'll be bored within the first hour, and the first complaint the FCC gets, they'll be down on you like a tonne of bricks.

Anyway, enjoy!!!