This is actually something we thought we fixed quite a while back. To look at it today, I took a random gccode from UK which had a high elevation, GC2CRWZ.
Our GetElevation routine has three different services it relies on. SRTM3 is the first, it returns false for GC2CRWZ. It then fallbacks to Google services, where we have a rate limit.
The third variant is AsterGDAM data, which is very jumpy. When it works, it's great, but when it's off, it's usually off with a lot.
A while back, we had another service between our local SRTM3 data and Google. That service isn't open anymore and therefore not used. We also had AsterGDAM before Google, which caused many issues.
When we moved AsterGDAM to the last in the routine we also ran an update on all geocaches we though affected. Either we missed a few, or we ran into rate limits with Google services so that our services fall back to AsterGDAM again.
We are currently running an update on all geocaches with latitude > 60 and elevation > 500. From what I can see, all in UK is already fixed.