In my personal Experience:
O2 - 3G coverage was useless, but around 3 months ago got much much better. When I don't have 3G EDGE isn't too bad for simple browsing, but GPRS is awful on o2, google.com will take 5 minutes to load. Customer service has always been great and from the one claim I've had to make their insurance department are great too. Data is unlimited and when you do get 3G it's pretty quick.
Vodafone - 3G coverage is great and when I was with them a few years ago it was fast. Customer service was always excellent and as with O2 the one claim I did make with their insurance was dealt with really well and straightforward. The reason I left Vodafone and won't go back is the data packages. Back when Nokia N95 the only data they could offer me was £7.50 a month for 300mb or something. This just wasn't enough really, as if you went over this allowance, it was very expensive per megabyte! Found a PAYG sim lately and wanted to try them out, but for PAYG data is was a complete rip off, suprise suprise.
3 - After Vodafone I then moved to three, purely because they offered nearly unlimited data for £5 and and apparently had the best 3G coverage. The data was pretty good, but when there was no 3G it was very slow, much slower than O2 EDGE is for me. When web browsing using 3 some websites are censored, I think this is still the case as I keep a 3 PAYG sim in my wallet for emergencies. It's not just naughty sites that are censored, everyday forums and sites I use wouldn't get past their filter either. It send you to a page telling you to sign up for adult access, but all it is, is their own adult sites and you still can't access certain sites. Other than that the data, calls and texts were all great. WARNING: Pray to god you never have an issue that requires you to ring up customer service. The people in the overseas call centres are pleasant and friendly, but generally, you can't understand them and they can't understand you. The flow chart they follow on their screen doesn't deal with many issues well either. I left 3 to join O2 to get an iPhone, they would call me everyday afterwards trying to talk me into resigning with them and offering me Nokia's telling me about the issues of iPhones. VERY annoying and I'll steer clear of them in the future. They are cheap for a reason! PAYG seems very good with them however and you shouldn't really have too many problems.
T-Mobile - Not used them for many years, they were always good though when I was with them, this was before the days of 3G however. The girlfriend is with them and has no problems, she has a HTC Desire and the web browsing seems pretty quick. Never tried PAYG with T-mobile.
Orange - Tried a PAYG sim a few months ago and couldn't get a 3G signal anywhere I went in my local area.
In a nutshell: If Vodafone offered a decent data package I'd be with them, their coverage is great, as is customer service. However due to data, which is more important to me than calls and texts, I'll stick with O2. They started off a bit patchy with coverage, but are getting better all the time.
