I think it's a fair price for what is an incredibly advanced piece of kit, providing they can make it do everything in the UK that it will in the US. If not, then it's about £80 too dear.
I jest, I reckon anything around the £400 mark would've been fine for either of the two consoles. Personally I'd like to have seen MS be brave enough to do a subs + cash model, maybe £200 and then signing up to XBL for X years.