Do those polls assume Brexit Party run a candidate in every constituency though?
They’d probably manage it if those poll numbers stuck. UKIP ran candidates everywhere in 2015 at their (lower) zenith (with a few random exceptions).
I thought before though Farage specifically allowed pro-Brexit Tories a free run at their constituency. I imagine he would do the same with some ERG elders (particularly the likes of Johnson who could lose his seat if BXP run there)
He has said they intend to stand in all 650 constituencies, they'd likely have the funds. Whether saying that is just designed to provoke the Brexiteer Tories into challenging May and supporting No Deal we'll see. He'd probably still prefer to pressure the Tories into leaving before any general election than having to contest them.
If it came to it I doubt many Tory MPs would be spared. After voting for the WA and rumours of potential leadership deals with Rudd, Boris wouldn't be one of them.