For £500 the rest of the car would need to be in excellent shape for it to be worthwhile. The car is probably not worth much more than that to be honest :-(
When you write something like that, people read it as "it's not worth the cost of repairing" without considering what you wrote properly.
I'd suggest there's a LOT more to think about before deciding to scrap a car and replace it.
If you take the £500 you need to spend on the Jazz and use it to buy a different car you could quite easily end up with a car that's going to need a lot of expensive or difficult work doing on it.
If you're DIYing, the Jazz is pretty easy to work on and if you're paying a mechanic you'll be paying for less hours of labour.
Also, the price of parts for a Jazz is ridiculously cheap compared to other cars.
Ironically, there are a lot of mechanical bits for Jazz's to be found in scrap yards as a result of cars being written off due to rust.
Obviously it's up to people to decide what their priorities are but I'd seriously think about spending the money to fix specific rust problems on a Jazz in order to continue taking advantage of cheap parts and easy labour instead of choosing a different car which might come with a bunch of new problems
and have more expensive parts and be harder to work on.
Course, if you've got a car which is just generally suffering from rust all over then you're probably fighting a losing battle.
In that case, maybe looking for
another Jazz, without the rust, would be the smart choice?