VFR UK: 2300ft under the LTMA, elsewhere I fly weird numbers like 3700ft, 4300ft, etc. Stay in Class G, usually. 1000ft above terrain, obviously (all VFR flights are planned and navigated as IFR).
VFR in Europe: Plan A is VMC on top, as high as one can, say FL100. Plan B is OCAS, which might be 1000ft or whatever.
IFR UK OCAS: as for VFR UK.
IFR (airways): In good conditions, file a flight plan for the lowest allowed Eurocontrol level (usually FL120, sometimes FL070 and sometimes FL150). In suspect conditions, file for FL180. In all cases, ask for a "stop climb" when VMC on top, to get best MPG and conserve oxygen.
There is an additional factor in VFR flights: you don't want to get into a corner at the destination, arriving above a thick overcast layer, and pretending to be VFR
So, one needs to descend early, somewhere out of the way of anybody, to be sure to arrive below the cloud. Unless the forecast for the destination is clearly CAVOK.