The price for a Brexit/Tory pact is the Tories committing to a no deal Brexit and not attempting to leave with a deal. At that point there is no difference between the two parties so where does that leave the 30% or so of Tory voters who are Remainers? Or Scottish Tories who support the Union something the Brexit Party would ditch in heartbeat? Come to think of it what is in this for the Tories? What do they get out of being tied to a single issue party?
Also a GE may start out about Brexit but will end up on austerity, NHS, social care and all the other issues. The last GE was called to give May a Brexit mandate. Labour talked about everything but Brexit. Single issue elections always get sidetracked into other issues.