My experience (some quite old) and impressions of the mobile providers, not to say it's all true:
O2 - very "businessy", and indeed seem the sensible choice for businesses. You get all the usual stuff like an account manager, SLA etc. Very blunt and to the point on what you can and can't do - they don't try to compete too much, or bend over to the will of the Average Consumer. A very good match for Apple in that way. 3G coverage almost non-existent.
T-mobile - never been with them, worst general coverage let alone 3G. Cheap as chips, which is why the likes of Virgin and other rebrands use them, but you get what you pay for. Very much a "pay as you go" type network.
Orange - fair balance between the networks, though seem a bit "new age" in how they portray themselves. Not really one I'd pick for a business. Very little 3G coverage, but better than O2's (not difficult). Not had much experience with them.
Vodafone - "I'm the individual consumer's provider with the cute phrases". Obsessed with branding everything: if the iPhone was with Vodafone you'd get a red background, red menus, poor quality red icons, the home button would be red, and it'd have the (red) logo stamped on the front. Fairly decent as a provider. Great shops with helpful staff. Second best for 3G coverage.
Three - Unrivaled contracts, you can get sickening amounts of texts or minutes, and they seemed to kick-start the "calls mobiles on other networks count in your free minutes" thing that we take for granted nowadays. Pioneers of 3G with the coverage to prove it. Falls back onto O2, or I believe with new contracts it's Orange, when 3G is not available. Brilliant portal, or you can use the internet. Other sweeteners such as Skype, which makes a voice call to a gateway number - very clever. But atrocious customer service using clueless Indian call centres.
If you don't want an iPhone, take your pick really! At the moment I'm thinking O2 and Orange are the lesser of the evils, or Voda if you want 3G, or Three if you want 3G really really badly.