How would you determine the base of controlled airspace defined as an altitude? Maybe some don't care enough to bother? I remember having my Silver height attempt disallowed years ago because the instructor worked out, with the prevailing Regional QNH, I would technically have JUST been in controlled airspace - not so if I based the climb on the pressure setting for the airfield at the time (a bit tight that I always thought?!).
I might have expected this conversation on a private flying forum from an inexperienced pilot asking a genuine question due to lack of understanding, but on a forum inhabited by 'professional' pilots I find the subject surprising.
SpannerInTheWerks, others may find it surprising you don't seem to understand that the base of controlled airspace defined by an altitude is with respect to the QNH of an aerodrome situated beneath the controlled airspace, not with respect to the RPS! AIP ENR 1.7 refers.
Or perhaps you do, and I too have misunderstood your post. Perhaps you'd care to clarify the answer you had in mind for your own question:
If you're flying from a private site, that is one without ATC, which QNH value would you use to fly cross-country?
One derived from the aircraft altimeter 'adjusted' from the airfield elevation setting, one published on the 'web' before flight or the Regional QNH obtained from ATC?
And in the process, explain how your answer is supported by your curious footnote:
PS: Not to mention 'collision avoidance' issues!