There's also a legal requirement to make sure it's approved. I don't think it was ever offered as a UK dealer option, so, even if you get one fitted from parts supplied by a Company in Europe, it may still be illegal to use it, so do your homework before splashing the cash.
http://home.btconnect.com/stanleytowbars/thelaw.htm
The important bits...
1. Fitment of a non Type Approved towbar for a vehicle registered 'S' and onwards could result in prosecution.
2. If a non Type Approved towbar is fitted, the driver's insurance may be invalidated in the event of an accident.