If you search there are loads of threads on this. But yes, HD-DVD's in general are multi region so will play on any machine be it US or UK. However, the HD player is region coded by the 360 itself. So even though i have a US one, i can only play region 2 SD-DVD's. There is talk of HD-DVD's being region coded around the 2010 mark, so quite a while to wait yet.
If you are thinking of importing one, Movietyme are a good bet.