We don't get 4G girl as 4G in America is HSPDA, a 3G extension. GG americans.
Here are your options for buying a phone and some background:
Buy from 3rd party whom the network gave £XXX bonus to (to acquire you), to get you to sign up. 3rd party gives you some of this back back (redemption, xbox etc...), keeps the rest for profit, everyone is happy. Customer is double happy if they can stack a double (cashback + redemption)
Buy from 1st party who will hopefully reward your loyalty/purchase with a discount like Oranges 10% renewal discount but seldom offer any other incentives. Their costs are low when you do this, they are happy. Hopefully you are too because you got a decent offer.
THE PROBLEM: 1st party notices you're tight fisted after 24 months and don't go over your free minutes, work out you're not so fun to be with / give minimal profit so at renewal they make it not so fun to re-sign by upping the deal in the networks favour or at best - same as new customers deal. You either sign up or leave, if you leave you probably will cause the same issue at another network so it's now their problem.
The problem for the networks when you leave is that you may just go around the block and re-sign up again with the 3rd party (who they give even MORE cash to) but stay on the same network which is why they make it oh-so difficult to do PAYM-PAYM number porting, no one likes effort when you can just re-sign up which is why loyalty is a carefully balanced seesaw, you can't be too mean (profitable people run off) or too kind (you get less profit).
3rd parties like cpw/p4u are the bane of the networks lives. If they could kill them all, they would.