Have I missed anything ?
pb84,
I would add:
Don't fly along the centre of a valley - fly at the side to leave more room for a turn back - preferably on the right side.
Always cross a ridge-line at an angle and be prepared to turn away ( as you say, "always have an exit").
Get PIREPS if possible. Pilots flying the VFR routes (Ve Follow Roads/Railways) in the Canadian Rockies often give reports on the weather at the mountain passes.
Have some kind of PLB (and don't leave it in the glove-box of your car!
)
sapperkenno,
I accept that it may get hot in the UK occasionally, but none of the airfields you mentioned qualify as "high". I routinely operate out of a grass-strip that can have density-altitudes of over 6000' in the summer and that's in the flatlands, not even in the mountains.