More information: Consultation on the Policy to Introduce a Harmonised Transition Altitude of 18 000 ft in the London and Scottish Flight Information Regions.
http://www.caa.co.uk/docs/2257/20120...onDocument.pdf
I would have presumed that we would have maintained quadrantal levels up to 18,000 but there is a suggestion of implementation of a new semi circular altitude based cruising level system outside CAS.