What is missed is you cannot win an election with only the votes of one side of the Brexit debate. For this strategy to work the Tories would need to fight out off the BXP for leave votes and they would have to win leave votes in big numbers in the Cities that voted remain. The Labour heartlands which voted leave are more likely to vote BXP than Tory simply because the Tories are so hated in many Northern labour strongholds or will revert to type and vote Labour. The Tories will also have to hold on to remain voting seats which could prove very difficult because Tory remain voters may turn to the Lib Dems.
It is also assumed that people will vote along Brexit lines and if so that will still leave around 30% of the electorate who didn't vote either way in the referendum. Its their votes that will be targeted and they will decide the outcome. In my opinion this is where Labour will be strong as their messages on the NHS and rebalancing the economy will attract many of those voters because the Tories will be trying to out Brexit the BXP on Brexit and there vision will be narrower. That is why Johnson is so desperate to get a Queens speech so he can try and get out a different message that Tories have other policies rather than just leave, leave , leave, deal, no deal, deal, leave.
The lead may be 20% atm buts that Brexit influenced and in GE I am not sure Brexit will have the same influence simply because the electorate are getting fed up with the whole Brexit fiasco and the Tories will suffer for it.
Labour and the Lib Dems have far more capable activists out on the ground doing the door knocking, Labour has far more members for a start and that will make a difference, Corbyn is better at electioneering than he is in the HoC and its yet to be seen if Johnsons affable buffoon act will convince on the hustings.
The Tories also face the possibility they will be wiped out in Scotland, the DUP might suffer in NI and Plaid may make gains in Wales as regional nationalism inspired by English nationalism grows stronger.
It will be one of the most fascinating GEs ever that is for sure and the 20% I do not believe has any bearing on the outcome, because as a recession approaches Bill Clinton's adage about "its the economy stupid" becomes more relevant. If the economy crashes under the current government and the indicators are strong that it will then the whole austerity narrative will be shown up as a sham and it is game on.