Good points Nick.
Looking through the AICs for this year's airshows, the top of the display airspace seems to be generally around 5500' QNH.
When I did my PPL course, that was generally accepted to be sufficient for aerobatics in a Cessna Aerobat.
When we did the shows at EGCD, I'm sure any fast-jets were given something like FL80-.
The Red Arrows airspace at EGXP, (R313), is SFC-9500'.