The Nexus One can be bought now, but you buy it from the Google.com/phone US website. They ship to the US, UK, Hong Kong and Singapore.
It will be available for purchase within the UK in the next month or so hopefully.
Windows Mobile 6.5 is a fairly decent OS, but there are still things about it that will drive you mental. Read the HD2 thread in the HTC forum and see what I mean... start from the middle of the thread to save time, but you'll see plenty of references to registry changes, patches, fixes and other such software changes.
Some people love this sort of thing, the customisation and software hacking and cracking. But if you liked the iPhone because everything just worked right out of the box, forget it.