They all have reasonably good coverage in London. O2 and T-Mobile/Orange have the most extensive networks and thus possibly slightly better coverage than the others, but they all have occasional black spots too.

On the price side they all have pretty similar pay-as-you-go deals, with £15/mo or so getting you a bunch of minutes, txts, and 500MB-ish data.

Orange will get you 2-for-1 cinema tickets on Wednesdays, and O2 now have a similar (50% off) cinema deal which is good Sunday-Thursday, but only at Odeon cinemas.

Best bet is to check out the PAYG offers on the various carrier's sites:

www.o2.co.uk
www.vodafone.co.uk
www.orange.co.uk
www.tmobile.co.uk
www.virginmobile.co.uk
www.three.co.uk

(note that Orange and T-Mobile are now actually the same company, and Virgin share their network)

Or, just pick the first one you see and don't worry about it