Perhaps I was unduly harsh in my post.
However, a quick google search came up with this
https://www.gov.uk/national-minimum-wage-rates showing minimum wage at £3.68 for your age.
At your age I was working for £3.28 an hour, and having national insurance deducted so it worked out even less.
So you have to decide whether you want to go out and find a real job, which probably won't be any better than washing dishes, will be longer hours and more of a commitment, or whether you are happy with this "volunteer" agreement. Still not entirely sure of the legality of this arrangement, but they have been clear what they expect from you, so the ball is firmly in your court as to whether you want to put in the graft or look elsewhere.
If I were in your shoes I would persevere with the volunteer programme. It's going to get you exposure to the flying club environment, and you will meet a lot of pilots, instructors and owners who will ultimately be able to take you flying on your days off once you've built a friendship with them.
Also, if you are able to impress them you may well find yourself able to move on to better things, or when you turn 18 getting a proper job with them behind the bar or whatever they have.