This is a deal that is not advertised anywhere, but if you ask for it then the store has to match the online price. For example, if a phone is FREE at £30 a month online, but costs £79.99 at £30/month in store, then the store will have you give you the phone for free. This is only if you ask for it.
I thought this might be useful to anyone who wants to get a phone online, but needs it before Christmas. Now you can pick it up in store!
But also remember that buying online covers you with Distance Selling Act 2000 i.e. you can return a contract within 14 days providing that the SIM card is still in its plastic and the phone has no evidence of use. As long as you wipe the phone after you're done trying it out then you can return it.