SASless - You should fly on QFE within a MATZ because its dimensions are based on the airfield datum and all circuit traffic is using QFE. Regardless of ATC instructions the pilot is responsible for terrain clearance (as with F15s) not ATC.
Not quite sure why so much of the world has a problem with the concept that an altimeter reads zero when you land (using QFE) it's not quaint just because you don't like it.
Cyclically - most MATZ circuit traffic is at 1000' QFE so a standard clearance would be not above 500' QFE to give adequate separation from circuit traffic.
If you want 600' or more then ask for it in your initial call - ATC are not clairvoyant. ie - request MATZ penetration at whatever height on QFE that you think will keep you 500' agl or more inside the MATZ.
If you cannot get the height you need due to circuit traffic then either wait, go around the MATZ or accept that you are flying in compliance with ATC instructions and are unlikely to be reported or prosecuted for illegal low flying.