VHG has almost certainly identified the problem. Just because an 'HD Ready' TV set has built-in Freeview it doesn't mean that it can receive Freeview in HD. It simply means that it can accept HD signals from an external source (such as a Blu-ray player, a Freesat HD box or a Freeview HD box). The tuner inside the TV set is just an 'ordinary' (SD) one, not an HD tuner.
Unless you're able to exchange your TV set, you'll need to buy a set-top box in order to receive Freeview in HD:
Chris