Islander2
I completely understand that the actual depth of the layer will vary on the QNH. I simply had it in my head that there was an absolute minimum of FL40. It may be that this is a figure I have heard abroad.
Personally, I would question the wisdom switching to levels for cruising at 3000ft in a country where the spot heights extend to over 4400ft. If the pressure drops you could find yourself rather closer to the ground than you were expecting. It is much easier to reset the QNH periodically than constantly trying to calculate your actual altitude whenever there is a change in the QNH. It is fine if you're flying in places like East Anglia or the South East but I prefer to stay on the Regional QNH for VFR. Bear in mind that the higher levels are rarely an option around the higher ground in the U.K. if you want to remain VFR.