Happens around the whole TMA area. (Bandit - there is no Heathrow CTA mate)
Although, the Manual of Air Traffic Services pt 1 (CAP493) says:
Except when aircraft are leaving controlled airspace by descent, controllers should not
normally allocate a level to an aircraft which provides less than 500 feet vertical
separation above the base of a control area or airway. This will provide some vertical
separation from aircraft operating beneath the base of controlled airspace.