Little bit of google...potentially you can go to a max of 245 on an 8" rim. Your options are the 235/40/18 or 245/35/18 both of which give a 1.2% difference in rolling diameter which is liveable with in terms of your speedo, don't know if this would upset the ABS sensors or not. SO you could go 10mm wider on a 235, and 5mm inner/outer increase shouldn't give you rubbing issues. Kinda expensive experiment though.
Thank god stretching ridiculously under sized tyres on 10" wide rims has faded away, they've probably all written off their cars.