As people say it's operator/regulator dependant - for the sake of the question about HKG-LHR the basic rule set for one of the 747-200 operators operating too/from, the far east in the late 80s was, at the planning stage..loading /offloading SLF stage
At start Fuel in tanks >= the sum of:
Taxi (out) fuel.
Trip fuel ( departure to destination)
Contingency fuel ( usually 5% of trip fuel)
Diversion fuel ( typically LHR-LGW from the go-around, if LGW weather was OK).
30 min reserve...
Reclearance was an option, routine use of 3% contingency by use of a nominated en-route alternate came in I think in the 90's...
A much current 747-200 operator will be more up to date on the exact numbers for fuel than I now am but I'd have a rough punt from memory that your 30 min reserve could be 5-6 tonnes, div fuel (LHR-LGW) ?3 or ?4 more.. .very open to correction.