Never known them to buy out your existing contract but at my local store, as soon as your through the door, they are immediately doing their best to sell you a contract. This is even after you have already explained to them that you are already in contract with another network and only browsing and wanting to know what deals they could do you. I think they used to give £100 for any phone that you no longer wanted if you taken out a contract with them. It might be worth mentioning that you would take a contract with them right now if they are willing to buy out your existing contract or pay so much towards it. Obviously that would depend on how much of the contract is left.
Not known any O2 store to be quite like that. Though support over the phone when you call one of their stores is not quite so good but customer services is first class.
Personally, you would be best to let your current contract run out and go to an individual network such as O2. You will get far more benefit from joining them than I'm sure you would in the long term with joining CPW.
O2 Retentions are always very good with their offers.