I agree about the Haslam school, that is too expensive too but 425 quid for CSS is a joke. Like you I did level 1 and 2 and they were good days but too expensive then, 5 years ago, at 300 quid each.
If you do all 4 levels at CSS that is 1700 quid FFS!! The US version is half the price, you could, near enough, have a weeks holiday in California and do all 4 levels for the same money as 4 days spent at Silverstone.
What you would learn in 4 days at CSS could be learned and exceeded by a race coach in 1. That costs from 250 - 600 quid depending on who you get plus you will go faster there and then, pretty much from lap 1 session 1. CSS dont teach trial braking and all of their days are run at less than trackday pace. TBH the best thing about CSS, possibly the only thing worth the money, was Andy Ibbot and he wont be there this year.