Soldato
That 10 quid style insurance that Silverstone use might be applicable here?
Never heard of it - I have a £1000 car I use as a track day car so whilst it's advantageous not to crash it, it's not world ending if I do. Likewise is someone else hits me - I'd be ****** off but not hugely out of pocket.
Especially if it's just body damage - buy a cheap **** 318 and move all the parts over

The cost of a 500 quid car + a garage to store it in would probably cost you less over the year than a couple of rentals - might be worth looking into.
Especially if you can get a car with a desirable interior worth selling.