No direct experience but I'm fairly sure that as long as your seat/carrier carries US DOT approval for use in a motor vehicle, that approval will be honored in the UK.
http://www.childcarseats.org.uk/law/index.htm
Though the site only addresses car seats
sold in the UK must meet one of the available standards.
You could try emailing UK DOT:
[email protected].
Due to the large number of emails we are receiving in response to the change in child car seat legislation it is currently taking up to two weeks to respond to all enquiries. We are sorry for the inconvenience that this causes