A certainty in politics you say? And what's more, alleged certainty of outcome in what is clearly the most uncertain of times in UK politics for perhaps 100 years. I think, not!
To repeat (sorry about that!) - if Boris becomes PM (looks highly likely at present ... but almost any surprise could happen yet; especially with Boris!), then it's very likely that he will get no new deal from the EU, and he will as a result be forced to offer the HoC a No Deal Exit ... on all evidence so far, that option would be rejected by MP's (though it could be very close) ... in that case Boris begins to look to the Conservative MP's like an even greater liability and an even bigger embarrassment than Mrs May in her very worst moments ... that may easily lead to a General Election ...
... in that case, Farage has already sworn that he will field a nationwide set of candidates for his Brexit Party.
If you think the Brexit party cannot possibly win, then you are an absolutely awful judge of politics lol!
Farage & his Brexit Party may not be the favourites in a GE, but they already showed that they can count on about 30% of the electorate as absolutely insistent on voting for Farage regardless of any other policies or any other issues at all. That, by any parties standards, is a very big threat indeed ...
... and they do not need anything else at all in a Manifesto. For Farage, the Brexit party, and about 30% of the electorate, this would be a one-issue election.
30% may not be enough to give Farage victory, but he would probably only need to pick up another 6% or 7% (something like that … keep in mind, that would be 6% -7% votes gone from any of the other parties).
Personally as I said above, I think the most likely result would be a narrowish Labour victory, with say a Majority of maybe 15 seats. But another obvious possibility is that Labour and Lib Dem could agree on an anti-Brexit coalition, with their votes added together to give a majority of maybe about 30 or 40.