Just having them on the ballot will put a big dent in the tory pro brexit vote, maybe only 5 - 15% of any vote share but enough to kill off any Tory surge. Farage has never done ground level campaigning - that is in part why he has never won a single seat. He does however run a strong central campaign based around a brand and a single policy. At the next GE the harder he pushes the more votes BoJo/tories lose - it is that simple.
I can see that the Brexit Party would take votes in some Tory seats without really campaigning. But those will mainly be in the very safe Tory seats where the Colonel Blimps reside.