No it was not !.
Manchester Airguns developed the MAG Mercury Magnum which was a BSA Mercury Mk 3 available with .177. .20, 22, .25 barrels in 1979 / 1980.
The .20 and .25 barrels were opened up from .177 and .22 barrels by Gerald Cardew.
BSA themselves offered the . 25 BSA 635 Mercury around 1988. They also then started offering other models with a .25 option
barrel.
M&G offered the model 45 in .20 and HW had .20 and .25 barrel options to special order in the early 1980's.