Bookworm,
FL31 will never come into it because it is not a flight level used in the table of crusing levels.
The 500ft vertical separation between levels is universal. The only difference being that the UK has a "quadrantal" use for it while other places reserve the intermediate 500ft levels for VFR flights and the whole thousands for IFR flights.
In accordance with ICAO altimeter setting procedures which every PPL should have been taught as it is part of the JAR-FCL sylabus, the transition level is defined as the lowest flight level available for use above the transition altitude.
ICAO leaves two options for countries when choosing this level. Countries can require vertical separation between an aircraft at the transition level and the transition altitude and thus the transition layer will be a minimum of 1000ft. With this system you can indeed have a transition level of FL45 (i.e. an intermediate 500ft) however, in such a situation, ATC will often use a whole FL as the minimum crusing or holding level. Otherwise with the requirement to provide 1000ft vertical between IFR flights, the 500ft gets pushed up and up the stack and the posibility of an error.
The other option wjich the UK uses is to not provide any vertical separation between the TL and the TA and simply require the TL to be above the TA. The UK then uses minimum stack levels or minimum cruise levels to provide vertical separation when required.
Overall as I said previously, the TL is meaningless for most pilots. If you are climbing to a flight level then you set the altimeter to standard setting when cleared so that you do not forget and when cleared to descend to an altitude you set the QNH again so that you do not forget. The exceptions being when asked to report passsing an altitude in the climb or flight level in the descent.
Whoever has a problem with a 3000ft transition altitude when the higest obstacle is over 4000ft needs to remember the following;
1. There is a UK AIC with a Terrain Clearance table that you can print off.
2. You also have to factor in wind effects and temperature corrections as appropriate thus even using the QNH for minimum safe vertivcal separation from such an obstacle is not simply a case to making the altimeter set to an appropriate QNH read more than the altitude of the obstacle!
There is a graph in the UK AIP that you can enter with the QNH and altitude you want to fly at to find the minimum flight level that satisfies that requirement. You can use it to find the level that is 500ft above the transition altitude or to find the level that is 2000ft above your local Munroe.
I recomend that all IFR fliers in the UK have the terrain clearance table from the AIC avaiable in flight.
Regards,
DFC