I certainly would not try exiting LGW or anywhere else via a R.E.T. (rapid exit turn) at the maximum 60 knots in a 747 or other wide-bodied type. Whilest the use of reverse is not factored into certification, speedbrakes or more correctly, ground-spoilers, are. From the Boeing Flight Crew Training Manual for the 747-400 "unless speedbrakes are raised after touchdown, braking effectiveness may be reduced initially as much as 60%".
As the old adage goes "bang it in, stand on them and slam them into reverse!" then leave the runway quietly and wait for the complaints from the cabin crew!!