pembroke, from speaking to a couple of TMA controllers, you can go to 5500' and be safe from the airway as they will not use a flight level that is not separated from 6000'. Still searching for a reference to confirm this so don't accept this as gospel just yet.
Jim59 - more information for the transition altitude change can be found both in
ATC Issues and
Rumours and News as discussed early last year.
Zooker
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.
It's not a UK thing, but a European wide harmonisation. The benefits could be huge in cutting costs as several posts in the links above detail. NATS are still working extensively on introducing 18,000ft with an open mind that this may not quite be the final decision. The LAMP project (point merge/tromboning approaches amongst other changes to the London TMA) is also working with a much higher TA in mind than the current 6000ft.