The usual reason is simple economics. If fuel is more costly at the destination it may be desirable to tanker to max weight ( landing or or Take off ) to save money on the overall round trip.
Simple formula is about 4% per hour of climb and cruise. On LGW TFS the flight time is around 4 hours of which 3.5 hours is climb and cruise. Thus 3.5 x 4% = 14%. So the fuel would need to cost 14% more at destination to start justifying the use of economy tankering. Obviously the higher the difference beyond 14% the greater cost saving.
There may be other factors to be taken into account beyond cost such as shortage of supplies at destination etc that may need to be taken into account.