To be totally clear you'll pay for the time you spend flying NOT a set charge per session.
What does concern me is you seem very much focused on the cost, which is understandable but you need to be clear that if you think you can
just afford what the school/club are advertising, then you possibly can't afford it. I appologise that that sounds presumptuous but unless you do a residential intensive 24/7 course it's highly unlikely you'll be on your skills test on hour 45. Landing fees may not be included and are not an insignificant cost, I'm learning out of Southend so believe me I know that first hand
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The books and stuff mount up and you'll soon be wanting your own headset. Add to that travel costs and it's a damn expensive hobby.
As long as you're aware of the extras that don't tend to be shown near the headline price of the PPL then you won't be shocked, but don't get confused on how lessons are charged.