As Tom Allen has put - if a bottle is the old UK HSE spec then it can still be re-hydroed as required and is still uk legal
If not uk legal DOT bottles still could be hydro tested, but it does not make them uk legal.
Up until recently there were cases of people using US bottles, but would get around the rules it by filling a uk scuba tank on the sites fill rig and fill us bottles on the 'private' rig
The theory being the site were covered by legislation and not individuals (health and safety at work act doesn't cover individals)
Things have changed