I also think there is a ~ FL100 min crossing level for the LTMA.
However, it isn't practically all that useful for piston GA because one can zoom underneath at 2400ft without even filing a flight plan. I rarely file airways within/around the UK, but would do so if going say up to Scotland.
I do wonder what happens if somebody took off from say Denham, on an airways flight plan. They are likely to keep you low for a while. In any case, climbing to FL100+ is not going to be an instant proposition, short of an F16. I see a lot of Farnborough jet traffic kept low for quite some miles.