I'd too had understood that the PS4S wasn't available as an 18" in the UK
... but Blackcircles list a 215/35 R18 Y (84) tyre for £165
(this search might work for you too https://www.blackcircles.com/tyres/brands/michelin/pilot-sport-4-s#anchorAllSizes ). Which isn't quite what an Aero needs, but suggests Michelin *do* bring 18s into the UK

+ 18s are importable from the rest of the world - I've had stuff from DesertCart before, and they can be very competitive - but their 245/40ZR18/XL 97Y is listed at £574...
https://www.desertcart.co.uk/produc...ormance-radial-tire-245-40-zr-18-xl-97-y

I'm sure a rummage online would find a better price.

Will

Formerly Aero S5 #80
Currently 911 (992) Targa in python green