Labour had 2 full time coordinators in Peterborough from January. First job was to negate the criminal conviction of the former MP and second was to concentrate on local issues.To complicate things along came an unexpected EU election and the BPLtd. This not only had the effect of splitting the Tory vote but also getting some remainers over to us. Finally the GOTV was supplemented by 500 or so momentum activists bussed in from all over.
A one off election with an organisation that could not be replicated at a GE.
To answer your question, yes the BPLtd voters are mainly Tories with a few from other parties, including Labour, thrown in. Labour has a substantial group of voters who will vote for them irrespective of Brexit stance, more so than Tories.
Yes, Boris Johnson is likely to be elected as PM by the 100k or so Tory membership. A brexiteer whose reign will end in chaos.