I can't speak for the UK - where, as I understand it, glider pilots are regulated by the BGA and are unlicensed - but here in Canada (as in most other ICAO countries) a glider pilot has to sit a written examination to qualify for a pilot's license. Air law is definitely part of the examination, including the minimum vertical and horizontal separations from clouds.
The above being said, it is a fact of life that very few aircraft are equipped with extended yardsticks that allow pilots to accurately gauge how far away they are from clouds. Human nature being what it is, not infrequently pilots are inclined to overestimate the distances involved. That's not an excuse, but it is the reality.
MLS-12D
P.S. In Canadian uncontrolled airspace, the minimum distances from cloud for VFR traffic above 1000' agl are 2000' horizontally and 500' vertically. Sounds like your restrictions are more onerous than ours (no Finals Three Greens, I am not saying that this is necessarily bad; just different).
P.P.S. As I recall (???), cloud thermalling is (or used to be?) legal in the UK, although it is not in North America. Can someone from the BGA shed any light on this?