FWIW, Vodafone is the largest in the UK, followed by Orange, O2 and T-Mobile.
It must be remembered that our mobile phone culture is significantly more developed than that in the US right now. The whole "exclusive to a network" thing while not unheard of is certainly less common than over there, especially for a flagship model which the iPhone would be.
I'd also like to suggest that unlike in the US, where Apple is a real cultural name with history and gravitas, I can't see the non-UK networks bowing to Apple in terms of upgrading their network just for them. Look at it this way - why should they?
At this price, it's absolutely not a mass-market phone (and won't be for 12-18 months). I sincerely doubt that Apple will allow the phone to be branded, which means that the phone won't be subsidised. The phone is cool, definitely. But can you ever see something like this coming down to be a £100 PAYG model like, say, the RAZR did? I know I can't. Future models will be mass market I'm sure - can anyone say iPhone nano? - but this model is clearly designed for supercession, not trickling down the price spectrum like other models do.
Apple to the UK mass market isn't the pioneer it's seen as by the US, and us Mac users here are strictly in the minority. Apple's early successes in the computer business were pretty much confined to the US - we were more interested in the Commodore 64, Sinclair Spectrum in the early days, let's not forget. Until the iPod, Apple has never been a mainstream brand in the UK, merely "just another niche computer maker". I'm not sure the networks will be so keen to adapt to match Apple's technology. For Cingular in the US, it's a risk worth taking - Apple's brand historically has the kudos to pull it off. But to the UK market, it's going to be a niche phone from the people that made the iPod. So what?
So, my half-arsed analysis over, here's my prediction: the iPhone will make it onto most, if not all of the major networks, with perhaps one of them getting a 3-6 month exclusive period. The phone will ship without some of the features that would require network change, notably visual voicemail. Oh, and we'll see a stupid price conversion - I'm guessing £349.
NB: opinion, subjective, conjecture, etc.
If I'm wrong though, and it did have an exclusive carrier, I'd bank on Orange doing everything possible to get a European deal. They just seem to be the most interested in style, from their minimalist branding to their stylish stores, contrasting wildly with the likes of Vodafone. With knowledge of the French market too, Orange's brand is significantly more developed than SFR (linked with Vodafone) and Bouygues (cheap'n'crap brand owned by a building entrepreneur). A flagship exclusive like this would seem to be a perfect fit for them.