It isn't really that many. Conservative supporters have a high of about 40%, but not all of those will say they vote Conservative if asked (some change their vote from time to time and may describe themselves as undecided). But we will go with that as an upper limit. So 18% of 40% is about 7.2% of the population, so less than 1 in 11. That's still way too many, but a manageable number if it comes down to it.