HTC and mobile operators....
HTC only support handsets that they've sold directly, or via SIM-free routes. They will always tell operator-provided handset owners that their serial numbers are invalid because they are no longer responsible for the support and warranty of such devices. I had exactly the same issues with T-Mobile Vario variants of Hermes (V-II), Kaiser (V-III) and Raphael (V-IV, Touch Pro) handsets - and this is one of the reasons that many folk venture over to xda-developers, as mentioned by others.
So, T-Mobile has taken on all support and warranty of any T-Mobile branded devices, and similarly for Orange. HTC have nothing more to do with them.
Taking an image from ROM is not as simple as it seems though, if you want to revert things back to stock in case of warranty 'issues' - heres the link for Kaiser (Tytn-II, Tilt, etc) - http://forum.xda-developers.com/showthread.php?t=337066 - it's more than just running a device backup using SpB or Sprite, for example, as these don't image the firmware, just the device data and settings. Check the wiki on xda-dev, there's probably a base ROM already there.
Of all the operators, Orange seem to be best at keeping up with the HTC releases - after a while they'll stop but T-Mobile, O2 and Vodafone all seem to give up sooner.