Green’s Dictionary of Slang

(W.I.) the accelerator on a car.

1956		S. Selvon Lonely Londoners 27: Mahal was a mad Indian fellar who used to go around town playing as if he was driving car, putting in gear and stepping on the x and making hand signals and blowing horn.
1988		S. Selvon Eldorado West One 18: The drivers only have to see a spade crossing the road and all they have to do is step on the X and bam!