Insurance is a tricky one, as it's not only your own driving you're trying to cover, but also every other novice on track at the same time. You're only liable for damage to your own car remember! So if you crash into someone else, it's up to them to have insurance - unless they fancy taking it through the courts.
The level of cover you go for will determine the price. If you go for full cover (body, chassis, mechanicals, etc.) on a value of £25k, then you're looking at a £250 premium and an excess of £2.5k - and that only covers damage due to an accident, not if you blow the engine up by abusing it. However, if you go for body/chassis only cover of £12k then the premium & excess both come down by half.
I normally use
Competition Car Insurance or
MORIS.
Unless there is an accident, there should be no damage to your wheels (stay off the kerbs - using them won't make you faster until you know the circuit well, and they're more likely to make you unstable than fast) - you'll use a bit of rubber, especially if you 'over drive' the car (i.e. forcing it too much, aggressive inputs, etc.). If you've got other wheels & tyres then by all means swap them over, as it won't make much difference on a novice day, and if it's sessioned you'll only get about 3x20 minute sessions in 2.5 hours.