Depends where you get them from Sy.
I paid £420 for 4 225/45/17 but as you have 18's you will prob be around the £460-500 mark.
ATS Euromaster would be the place to go in Cheriton. They have an offer on with Michelin at the moment where you buy 4 Tyres but pay for 3. Also you get £40 of fuel vouchers if you send your receipt to them. Offer runs to end of May.
On Avergae though your looking at around £120-150 per tyre. Unles you go to sh!tfit and they wanted £186 a tyre