Brian Micklethwait argues that no (overt) pact is needed between the Brexit Party and the Conservatives in order to get a Parliament of leavers. The Brexit Party can make the pact unilaterally:
all Brexit voters need to know who to vote for in their particular constituency, come the day, to ensure Brexit. So, the Brexit Party just needs to tell them. If the Brexit Party campaigns for Conservative Brexiters who’ll win, but for its own candidates when they are more likely to win, the Brexit Party will get its deal.
Meanwhile Boris might do well to avoid a pact:
in the event of such public collaboration, there was and is a crucial slice of Conservative but only Leave-ish voters in the affluent south who would have been put off voting Leave, and would who would now be put off voting Conservative and would switch to the LibDems
It might just work.