The UK government says

"4. Towing an American caravan or trailer
American trailers and caravans don't always meet European safety regulations.

If you want to use an American caravan or trailer in the UK or the EU, you must first check that it's legal.

Download 'Department for Transport factsheet about towing American caravans or trailers' (PDF, 57KB)"

That comes from here: - https://www.gov.uk/towing-with-car/towing-an-american-caravan-or-trailer

The PDF factsheet is here - http://www.direct.gov.uk/prod_consum_dg/groups/dg_digitalassets/@dg/@en/@motor/documents/digitalasset/dg_200827.pdf