Re: good coverage Pah!
I'm not sure its always been like this, but these days I wouldn't go near an MVNO with a barge pole. My girlfriend had a Virgin SIM (pretty good years ago) for a couple of years; the signal was usually pretty lacklustre to start with, but anywhere near lunch time or rush hour in London and the chances of making or receiving a call were as close to zero as makes no difference, with 'network busy' or three rising tones if you were dialing out, an endless ring tone that never cut through to voicemail if you were calling her phone. Data was never, ever good; if we were together when I was on T-Mobile, I got decent data rates and she got close to nothing - 10 percent of mine at best. And those were supposedly the same network.
We're both now on Three (Voda for overseas) and generally get excellent coverage for voice and brilliant data speeds as long as we're in London, which is most of the time.