Some brands are licenced for sale on a regional basis, ie. north america, europe, etc. etc. some more refined than that, to sell something in the UK which is licenced for resale in the US would be known as what is "grey market", whereas the garment in question will be genuine, however the item would have been imported to a different area from where it was originally intended for sale, therefore it would not have came to the UK via the correct UK distribution channels, the sale of genuine grey market goods is very commonplace, alot more than many people may realise, though not strictly legal, many brand owners turn a blind eye to a degree as it still generates advertising for them in a round about way, but other 'stricter' brand owners could hit you with a cease and desist order which I would advise to take notice of should the situation arise as some brands have extremely strict distribution.