I've found O2 has improved greatly over the last few years. Gone from no signal to 4G in large parts of East and North Yorkshire. I no longer need to take a 3 SIM on holiday to Orkney either.
If my house was 4 stories high, we'd get excellent 4G. As it has just 2 stories, we have a pico-cell which provides full 3G signal.
That said, O2's customer service was so crap that we switched to a reseller (5 Bars), so we have someone in the UK to talk to when there's a problem.