Does not the USFS call it mountains at 8,000 and up?
Either definition would rule out mountain flying in the UK.
There are definitions set by the DOI and USFS--I do not have them at hand, but will find them later tonight---they DO however rule out logging mountain time in the UK as you say.
DOI refers to the definition contained in 14 CFR part 95 subpart B. USFS is higher. More to follow.