As high as I can, unless there is a reason not to...winds / short trip / airspace / weather. The UK has pretty varied airspace, for example you can get up to FL195 VFR over Wales, or only 2499' near London.
FL50-80 is a comfortable altitude in many parts of the UK due to airspace, you can fly most places at this altitude - FL100 is better (TAS) when there are no airspace concerns, and although we have a ceiling of 20,000' on our aeroplane we don't have 02 and until we get it then we keep below FL120.
When I fly in the USA I normally go at 11,000 / 12,000, again due to the O2 thing.