Wow, talk about a steb backwards, aren't t-mobile the only one worse than o2 (3, tesco and virgin discounted because they aren't proper providers, just re-sellers)
3 do have their own 3G network. They just fall back onto Orange and O2 2G when there's no 3G available (and drop the call every time they do it

)
Tesco, at least when I was on it 2/3 years ago was O2 (and bloody brilliant)
Virgin is T-Mobile I think.
Although their 3G signal is good I'm not desperate enough to see 3 get it (good gawd NO).. been there
I've been there too. Was thoroughly ripped off, abused by CS and mistreated AND the signal was *****. They're the reason I will now only sign sim only 30 day rolling contracts.
I'd love to see someone other than O2 get the iPhone (and I am very fond of O2 - they're who I moved to after 3 and it was a breath of fresh air) simply 'cause the contracts are ridiculous. And the price of the 3G is a bloody joke. As is the fact that you can't unlock your payg phone (although that's probably Apple's fault)
As for Orange and T-Mobile. My boyfriend's with Orange, and they seem not bad although I get better signal on O2 and T-Mobile than he does. T-Mobile is definitely they cheapest and best value as far as sim only deals go, and my signal's a constant 5 bars with them, whereas with O2 would drop a bit sometimes.
O2's CS was outstanding though (and, unlike T-Mobile's, free), and they did lovely things like let me give 3 days notice instead of 30 when I stopped my contract - "oh, we'd never keep charging you once you'd moved your PAC" was music to my ears.
In Edinburgh though I think it's really Vodafone that gives the absolute best signal, even in dodgy black spots like my middle of nowhere university.