Errrr......would it not be 600 AGL without any reference to an altimeter setting?
QFE (ancient and out of date quaint practice that is... would only be used for the single small spot/area on the aerodrome) would not be useful for determining true height above ground outside that location.
It would seem a QNH reference ( altitude requested providing the legal clearance required by the 500 foot rule) would be much more useful.
I assume ground elevation figures on UK charts/maps use height above Sea Level....thus a QNH altimeter setting would apply vice QFE.
The most immediate question is "how does one get nicked for violating the 500 foot rule in the situation described by the original post?"
In light of the F-15 crash following ATC providing an improper altitude clearance and the pointed criticism of the controller by the RAF review board....would not this clearance be improper?