Is it legal to buy from US ? Yes, but be very careful about delivery charges and be aware of the potential for added VAT and inspection charges when it hits the UK.
Also be aware that the copy of XP home they are selling is OEM, due to changes in the microsoft license click here this can only be installed on new systems, to upgrade an existing PC you need the full copy.
Does the US windows offer £ & € support, can it be configured for a UK format keyboard ? I don't know, but worth finding out.