Cost of 4* equivalent petrol in 1982 was £1.64 per gallon according to the AA.
Which would be useful if Black Buck was carried out in aircraft that used 4*. Perhaps they could have driven there in an old Ford Transit.
Aviation-grade kerosene fuel with icing inhibitor (F-34 FSII), as used in jet turbine aircraft including the Vulcan, Harriers etc, was priced between 0.95 and 0.96 cents a gallon during the Falklands War, which would have been approximately 50p a gallon at 1982 prices.