CPW use to use there own billing department for o2. But now these use o2 direct.The contracts are identical.. so it's whoever you want to deal with for billing issues etc.
Infact, do the iPhone CPW contracts not go direct through O2 anyway? Anyone get their iPhone from CPW? Do you get a CPW bill every month or is it fully O2 branded?
CPW would migrate you over to O2 direct anyway regardless of who you upgrade/buy from.
CPW don't deal with iphones except as an outlet for sales/upgrades onto.