Yup.
I've got a US HD DVD drive for my UK 360.
Works perfectly fine. Just needed to get a UK "figure of 8" lead for the power supply.
HD DVDs are currently totally region free.
If/when they do ever introduce region coding for HD DVDs, the drive would just take its region from the console as it does for DVDs, so no problem for the future.
As for buying HD DVDs...
I highly recommend Movietyme for import discs.
www.movietyme.com
I buy most of my discs from there, 'cos it tends to be far far cheaper than buying UK discs in e.g. HMV, Gamestation, etc. Plus many things come out a lot earlier in the US (like with the early days of DVD).
Buying UK discs...
Avoid the high street shops, game shops, music shops, etc. Rip off, IMO (common prices I see are £25!!).
Try
www.sendit.com, or just nip into ASDA (small selection, but v good prices).