That lip is not a problem on BS34's. It doesn't go all the way around in fact the front and back the lip only goes past the corner a few mm from the sides, so plenty of room for the float adjustment tool to be used flat on the gasket...............
Defiantly a problem if it was being used on the 38's, haven't tried it but just looking i would say its a no go trying to measure of the gasket..........I agree, a consideration and adjustment has to be made if measuring off the lip, but with a + or - of 2.5 mm, (on brass floats bs38's), that 1 mm is still going to be within spec...........