I am fascinated to read the above comments from our Australian and American colleagues.
In the UK the Transition Level is the first available Flight Level above the Transition Altitude; this makes the Transition Layer anything from 1 foot to 499 feet, but never more than that.
The TA and TL are clearly not separated, and are in place merely to denote the points at which reference is made to either Flight Level or altitude.
Within controlled airspace at least, altitudes are not used above the TL,and Flight Levels are not used below the TA.