Your profile says you're in Oxford - so I'm guessing that your local airport is Kidlington which is to be frank a pretty expensive place.
Not far north from there is Enstone which is a bit scruffier and less formal, but likely to cost a lot less. Cycle up there and visit the (I think) two light aeroplane and one microlight school and ask each to talk you through learning to fly, and sketch out the costs. You should get good answers, and also a chance to easily compare the costs of learning on different things. Phoning to make an appointment, or going on a wet cloudy day, might improve the chances of somebody having the time to spend with you.
A Cherokee 140 incidentally is an expensive, large, underpowered and ancient heap, quite probably older than your parents - albeit also perfectly safe and easy to fly. There are better options to learn on, and don't waste money learning on a 4-seater if you can avoid it; the learning experience will be no better than on a 2-seater such as a C152 or any microlight, but it will cost more.
G