Deal for the HTC is good if you desperately want one. I'd check you're happy with the size/weight first though... Are you used to having a 6 incher in your pocket?
Buying from Carphone Warehouse you're effectively buying SIM-only and financing the SIM-free phone, they just wrap it up into one purchase/payment.
It's true you might have to transfer your number to another network (£1 PAYG SIM) and then switch it back, especially buying from a third party. My parents had to do that once.
Always switch to SIM-only when the contract completes and the phone is paid off. Otherwise you're paying over the odds for a phone you already own!
You can see it's actually less faffing in the long-run to get on a SIM-only deal you're happy with and let it run, then just buy your phones as and when you need. Downside is you can't buy the latest super-duper flagship phones as they're always a rip-off SIM-free.
Only effort is in hunting down the best SIM-free deals but there's always good phones being discounted somewhere:
http://www.hotukdeals.com/mobiles/deals/hot