You may also want to check out this site.
HM Revenue & Customs: Banned or restricted goods
Should be some more direct contact details available should you have a query about importing plants/seeds/flasks.
and relevant legal requirements.
Apparently it may also easier to obtain a plant from the USA to the UK, through a UK Nursery which could be able to acquire the plant for you as they may be dealing with this sort of Paper Work on a regular basis. (It's apparently to do with conservation, to make sure it's not Black Market so to speak)
Haven't looked to buy outside the UK/Europe myself yet. I looked into it a bit and shuddered.