Avoid Carphone Warehouse at all costs.
My partner thought he'd upgrade at CPW since he was already with them
a simple delivery and contract extension turned into a nightmare of 2 (two) whole weeks without any cellphone as the number was cancelled by CPW. No replacement sims were sent. Truly bad service.
In the end he went straight into an O2 shop, cancelled his old CPW contract and started a new one with O2
Painful.
My advice would be go straight to Apple (which I did).