Big money for gameshows weren't really a thing until the 90s.
Was the lifting of prize limits on ITV in the early 1990s the start of the increase in money prizes for the BBC, as they had to compete with ITV for ratings.
The size of the jackpot is not proportional to ratings, you could offer a jackpot of £20m and it doesn't guarantee people will tune in to watch if if the rest of the offering is pants. The large range of shows in the 90s and 2000s where they wrapped other game show formats around the lottery draw itself sort of demonstrates this to an extent. A handful of them were successful and ran for a while (Dale Winton's In It To Win It was a good example of this, as it ran for years), while others were dire and crap for one reason or another and they were never seen again (Big Ticket for example, which was so unsuccessful that a) not enough people came forward if they won whatever the criteria was to get on the show, so the format had to be butchered, b) half the audience walked out during filming of said show, and c) it was a ratings disaster).
Anyway the main issue the BBC has with regards to prize money, which presumably comes out of the budget, is that its probably not seen as good and proper to give away very large amounts of cash, considering the source of that money. Its the sort of thing its very easy to get wrong and have daggers thrown at you, so a BBC show that gives away cash that would now be a BBC Studios Production you quite often end up with a format that's deliberately nobbled in some way to discourage if you like winning a decent amount of dosh. Weakest Link technically could give away £10k an episode but they never did, and the quick fire format of the show served to effectively cap the winnings - a decent team could rack up over £4k, and a crap team would do well to get a grand.
The indie productions seem to work differently, I can only presume its because they foot the winnings, and the BBC contributes towards the production cost? So if you're jammy enough to win £100k (as happened a few times on In It To Win It, which was made by 12 yards), presumably you're not going to have £100k of licence fee money thrown at Bob from Sandwich in Kent who happened to know something about the Khyber Pass.
Prior to the rule changes for ITV in the 1990s, the trend for gameshows was physical items in favour of cash, so you'd win things like a washing machine or a fridge freezer or a speedboat if you're on Bullseye or a holiday to somewhere - the value of the items were key and even then couldn't go over a certain amount. Yes you could win a few hundred pounds on the throw of a dart as it were, but it was more of an experience than anything else. These days you go on BGT to win it and change your life. You weren't going to do that on Sale of the Century or Wheel of Fortune.