Judging by the fact that carriers usually release such updates for their branded smartphones after unlocked devices have already received them, it's probably safe to assume that HTC Desire handsets bought in the UK SIM-free and unlocked will notify their owners that the update to Android 2.3 Gingerbread is available sometime during the month of April.
Also, we've previously heard that the HTC Desire, Desire HD, and Desire Z would get updated to Gingerbread after the Desire S had begun shipping. With thatalready happening, these tidbits of info line up nicely. As they do with the one claiming that the Desire HD will get updated in April. No word yet on the Desire Z specifically, though it seems pretty okay to assume that it too will start receiving the update sometime around April or May.
As for owners of the original HTC Desire gadget, in a few weeks your phones will receive a new lease of life, so rejoice.