.....and CAS FL base 'busts'
.....and terrain clearance 'busts' by pilots who forget to change from SPS to QNH descending through TL.....
Rationalisation would eliminate the majority of such errors because for various reasons the majority of piston GA flights seem to operate below 6 in the UK. During my time providing ATSOCA it was unusual to see pistons cruising above 6 in the open FIR especially when impeded by so many CAS stubs.
An aligned half-million TA 6 and below would probably be useful to low level operators, more so than the old (mixed-up) 5 and below half million.