But then you have to factor in National Insurance contributions, which also have a (different) threshold below which they don't cost anything, then cost you 12%

But then you have to factor in National Insurance contributions, which also have a (different) threshold below which they don't cost anything, then cost you 12% up to around the £50K mark, then cost you 2% over that. And even that isn't quite the whole story, because you'll also lose certain child-related benefits starting from £50K if you have kids, so your effective rate for the next £10K or so can work out much higher than 40%. Then there are the effects of not-unusual things like pension contributions and charitable donations, which can move things around a bit more. It's hardly a surprise that people find the system confusing!