So its about time to untie it from the local terrain. If you fly around below FL180 you're probably doing it for fun and your aircraft is probably slow enough so you will not be exhausted by setting the local QNH once an hour. The volume of GA in Europe is microscopic compared to the US, how do you think they cope? However changing from STD to QNH in the middle of your approach after glide intercept, then go missed to a FL again is just plain stupid.
It has to be higher than the terrain. If you want a world wide TA/TL, you'll need it to clear the highest terrain in the world, FL180 wouldn't work for The Himalayas!
It sounds like what you really want is for the TAs in the regions that you fly in to be higher, that's not unreasonable, go lobby the government or something.