It’s not illegal if the purchase price is set first. It’s only illegal if it’s backtracked after an initial advertising campaign. Even then proving it wouldn’t be easy due to hundreds of different levels of spec and tyres of service/warranty package.
As for the customer always pays, yes that’s what I’m saying in reply to Rob who said the manufacturer pays the finance. They don’t the customer does if it’s a 0%.
This is why I never ever offered any kind of finance as it’s a very grey and confusing area. It was always best to quote the best price and then let the customer sort out finances themselves. Sometimes you got paid by a finance house and sometimes by the customer.
It would be hard to prove but be clear its is illegal to offer 0% finance and then offer a discounted cash price to a customer