This looks to be the best pre order deal on the HTC desire out there at the moment. It works out as £540 for the phone and 2 year contract with 300 mins/tests and unlimited broadband from t-mobile.
Compared to the Nexus one which this phone is a near clone of you are getting the contract for not much extra if you are prepared to wait a couple of months.
The £15 and £25 a month contract also look attractive if you need more/less minutes.
They also have this phone available on vodaphone pre pay for £378.99 and unlocked for £429.99. HTC phones are generally quite easy to unlock and debrand so the payg phone is probably the better deal of those two.