Based simply on UKIP's 2015 GE result, where there is record of Farage support, then Heywood & Middleton, Rotherham, Dagenham & Rainham, Rother Valley, Hartlepool, G Grimsby, Wentworth & Dearne, Stoke N, Doncaster C, Dudley N, Bolton SE, Don Valley, Barnsley E all good picks
If you scroll down the list, though UKIP is still on 20%+, then you are looking at the likes of Makerfield, Kingston Hull East, Blyth Valley, Barking, Walsall North, South Shields, Barnsley Central, Houghton and Sunderland S, Ashfield
Fast forward to the 2019 EP elections and look at the actual Brexit Party vote then you really start to zoom in on seats like Hartlepool (51%), Thurrock (51% -is a Con seat but was Lab and is obvious target for Farage), Ashfield (49%), Redcar and Cleveland (49%), Bolsover (48%)
Some other contenders based on existing 2019 support for Brexit Party -> Dudley (48%), Barnsley (47%), Rother (47%), Bassetlaw (47%), Stoke (45%), Doncaster (45%), Middlesbrough (45%), Wakefield (44%), Walsall (44%)
Seems to me at least that if there 'was' some kind of deal then BXP would most likely target the small number of seats that are (a) v demographically pro-Leave (b) where UKIP finished 2nd in 2015, & (3) BXP had strong vote in EP 2019 i.e. established 'habit' of voting for Farage
And those aren't hard to find. In fact I'm looking at them.
