In 1958, ITV pulled its version of Twenty One almost immediately after contestant Stanley Armstrong claimed that he had been given "definite leads" to the answers. In 1960, this resulted in the Independent Television Authority's placement of a permanent winnings cap for ITV game shows of £1,000, which the Independent Broadcasting Authority increased to £6,000 in 1981 and £6,400 when the British version of The $64,000 Question premiered in 1990. The winnings cap was permanently eliminated by the Independent Television Commission in 1993.
Didn't the cap actually remain at £6,000 during the $64,000 Question's run, with the show only allowed to offer the £6,400 top prize every other episode?
Celebrity Squares was, I believe, the first new or returning ITV game show after the cap was abolished. However, Central and Grundy understandably treaded carefully, as the show's cars weren't worth more than £20,000, and most of them were British.
Indeed, just about every show before 1998 didn't go too far beyond the old limit (with the notable exception of Raise the Roof). And even in June '98, WoF's £20k cash prize was being promoted as the UK's biggest game show prize - despite Millionaire being just around the corner, and the National Lottery Big Ticket offering £100,000 amidst its awfulness:
"What is two plus two?" "Rather more than three." "Yes, but can you be more specific?" "The Belgians."