I'm with AC
In Oz...anything above 10,000 ft and lower than 11,000 is within the "Transition Altitude"....above 10,000 you are in the Flight Levels...hence this LSALT of 12,000 is really FL120, based on ISA 1013hpa/mb.
If the UK is the same as Oz...any aircraft at "12,000" would in fact have 1013 set in the Altimeter, hence that aircraft would be reading and at 12,000 AMSL.
There will be some errors within the aircraft and positioning etc....
Bet this question was drawn up by a Navigator !!!, who has lost her way.
Duck.
Happy Easter to you all.