It's funny how eugenics is widely accepted to be a bad thing when applied to humans, but for some reason "genetic purity" is still lauded for plants and animals.

The real threat to wildcats comes from humanity destroying their habitat (which is why the European wildcat, once endemic to the whole of Britain, is now only found in the north of Scotland). I'd personally imagine that hybridization is preferable to genetic extinction, as far as the wildcats themselves are concerned.

Also, like I said, Scottish wildcats are no longer found in the vast majority of Britain (through no fault of the domestic cat), so unless OP is in a particularly rural area of northern Scotland it's a completely moot point either way.