Perhaps also worth mentioning that if you do have a mis-hap on the Ring, you're potentially going to lose far more than just the car: At best you'd have damage to the scenery which I believe sits in the thousands of Euros range per section right now, and at worse you could have hundreds of thousands of Euros worth of liability towards others if you manage to take them out as well, given that you'll have no insurance for the Ring.
What I'm really getting at is that trying to save a few £K in case of an incident at the Ring is really a drop in the ocean compared to what the actual bill will be if you do have an incident, and as such you'd be better off just driving it in something you enjoy and just accept that if something bad happens, you're screwed. If you're that worried, an even better option would be to trade down to something still fun like the Zed, and use the spare cash left over to hire something from RSR and use that to drive round the Ring in. You've still got excesses to pay if you get it wrong, but at least you're limited as to how much you could have to pay if the worst comes to the worst.
I personally wouldn't consider swapping cars just to drive the Ring.