Lexplorer - it's true schools don't have enough money and support staff are generally the first to go, because primary schools cannot actually get rid of teachers, anyway, unless they are tiny schools with small classes (and thus totally financially unviable in this economic climate), and secondary schools which offer hardly any subjects will be slaughtered by Ofsted and parents will look elsewhere. However, this is not because of having to pay the odd £20 here and there in overtime (you're unlikely to get much, if any, more than minimum wage in overtime pay as an entitlement), it's because they cannot afford the basic salary.
Btw, unless someone is only recently employed, redundancy is itself pretty expensive and complicated, so schools will avoid going down that path, too, as absolutely long as possible - not out of niceness, out of cost factors... So don't be the person who saves them money by not claiming the overtime you are allowed, then leaving of your own accord because of the working conditions, and not bringing a claim for that, either. You aren't saving your job that way, or even delaying the inevitable. They won't be in a rush to make you redundant, because that is hugely more expensive than overtime pay! Recruitment to replace you also costs money, so you won't be replaced if you leave (then your colleagues can get your pay in overtime claims!). And a school would be idiotic to happily wave off a qualified teacher who is working on TA pay that they are getting away with asking to fulfil duties outside normal TA duties as a result, and replace her with a parent with no experience whatsoever.
If in your school, teachers are still insisting on getting their PPA time, despite the huge cost of this to the school, then they aren't letting their contracts be ripped up and chucked down the toilet, either, so why should you? Either they need you or they don't. If they didn't need you, you wouldn't be there.