"Fourth, there’s the turnout issue – the total turnout in the European elections was 17.2 million people, about 36%. Just about half the turnout for the Brexit referendum (72%). Extrapolating that we are a “remain nation”, as Toynbee argued, and as Vince Cable said on Channel 4 news, from 1/3 of the population is absurd."
I don't have the numbers for all potential uk voters but I had a number in some notes indicating that 6m people could have voted if they had been registered. So in addition to the 46m who were registered the total who could have voted were around 52m. Given that only around 17,200,000 actually voted this is a share of the total electorate of less than 1/3.