Uh they robbed you. They need to pay at least minimum wage in the UK even for on call. You are also allowed rest breaks. What they did was unbelievably criminal. Hell if that overtime included times where you were asleep and you were still on call they still need to pay you the National minimum wage for those hours as well.

Only part that wasn’t illegal is the extra restrictions, as you are still working so you can’t exactly treat it as a day off.