A transition Altitude of 6000' seems the best option to me, given that the highest obstacle in U.K. airspace is 4409'.
A TA of 18,000' is just below the level of many current area-sector interfaces. O.K., that could be changed, but as you can lose 2 flight-levels, it would require significant 'airspace engineering'.
Also in the U.K. we have quite deep low-pressure areas transiting the country, as recent events have demonstrated.
The steep pressure-gradients associated with these events cause significant rapid horizontal changes in QNH.
If you take an aircraft descending from FL200+ into the EGCC TMA, the QNH on passing the 18,000 TA (say, near EGNX), could easily be 4mb/hPa different from that required on final approach.
Also, The U.K. has a lot of traffic cruising around FL180/FL200. This would have a major impact on the availability of those levels.
In January 1995, I was involved in the EGCC evaluation of MIRSI/ROSUN and Runway 2 procedures at Bretigny-sur-orge. Before we set off, our ops manager gave me a paper to read concerning the planned withdrawal of Altimeter Setting Regions, and their replacement with local QNH.The main thrust of the argument for this was the requirement for a meteorologist to forecast RPS, and the associated cost. Fortunately, 21 years later, ASRs are alive and well, and said meteorologist still has a job.
Prior to that, there was CCF, another priceless failure from The Ministry Of Bright Ideas.
