| Vodafone and O2 don't generally deal direct with customers, unless you're a big business. They deal through Service Providers in the main. Carphone Warehouse being an example of one of their SP's. You get your phone bill from the SP if you're on a contract.
Three and Orange tend to deal direct with the customers. They bill the customers directly themselves.
One2One (or Tmobile as they call themselves) which are also Virgin Mobile and Easymobile tend to do a bit of both.
You'll find that you'll get the best deal on the high street mainly for the handset prices / contract deals, apart from Orange where the prices tend to be the same wherever you look. If you talk to the network they'll charge you the cost of the handset at Sim Free cost versus the SP's who'll get the subsidy and the kickback from the network for the connection cost.
You also don't necessarily need to stay loyal to O2 to keep your number - if another network has a better tarriff deal, then move to them. You can get a PAC code from O2 to move your number to a different service provider if you wish.
Mark.
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