Depending on condition and whether it's unlocked, CEX will give you in the region of £185 (locked, grade C) to £286 (unlocked, grade A). You'll get about 11% more than that if you go for store credit instead of cash.
In terms of store credit, they sell the HD2 for about £400 locked, £470 unlocked, again varying depending on condition and also on network if not unlocked.