The highest elevation in the UK FIRs is Ben Nevis, 4,409' amsl.
Therefore there is no requirement for an 18,000' transition altitude. 6000' would work well in UK/FAB airspace. It works in the LTMA, some of the busiest and most professionally controlled airspace in the world.
OK, you could possibly argue for 8000' over Scotland if the pressure was really low. I don't know why the Irish want 18,000 as Lugnaquilla is over 1000' lower than Ben Nevis.
Forget transition-levels also, and the transition-layer for that matter.
What matters is the lowest flight level providing a MINIMUM separation of 1000' from aircraft operating at the Transition altitude of 6000'. It's often called 'Minimum Stack level'.
"Separation standards are minima".
Much of the en-route sector structure involves interfaces at FL195, therefore major engineering would be required, by organisations which seem to be focussed on reducing costs.
Last edited by ZOOKER; 20th May 2013 at 21:00.