Most warranties on older/cheaper cars are minimal - LOTS of exclusions, you also end up suffering from 'betterment' (basically, if something breaks, and it is covered, you are expected to pay part of the bill, as your car is now (allegedly) better than it was before the part failed.

For example, warrantydirect (one of the better, and thus more expensive companies) have 4 levels of cover - if your car is over 6 years old or has done over 62k miles then you can ONLY get the 2 lowest levels of cover.

There are a lot of cheaper companies than them, but their exclusions, etc will probably be a lot worse.

In general, if you are buying a car for £1500, then it will be coming towards the end of it's life. Major parts will probably be wearing out, the car will be older, and will have probably done considerable mileage. All those things, and most warranty companies won't want to know - after all, they make money by paying out less than they take in. Giving cover on what is essentially an old banger does not make economic sense to them.

You already recognise that at the price-level you are considering, cars can be a lottery. If I was buying, I would certainly look at private, as a dealer will usually expect about £1000 more, like for like. Some nice ones should be available if you are willing to travel around a bit.

But a warranty at that age? You're just giving money away - swapping a certain 'reasonable' payment for a good chance of hardly any cost above normal maintenance, if you find a good car. Put the warranty money in a pot and use it if repairs are needed. Take your time and buy carefully.