I am wondering if political parties concentrate all their efforts on marginal seats. if you live in a safe-seat or in a strongly one-particular-party voting area, (I live in a labour voting area but right next to me to the West is a Tory voting area and about 300 metres to the South is another Tory voting area) I don't think that parties make much effort. I've only ever had the Labour Party knock on my door in all the years I've lived here. I live in a safe Labour voting area.
to some extent, yes. parties will focus their efforts on marginal / target seats, depending on how the polls are looking they may concentrate on defensive marginals (ones they are trying to hang on to but not confident) or target seats that they hope they can win.
as things currently stand, labour / lib dems are putting an effort in to seats that they wouldn't have made much of an effort for in the past, and the tories are putting an effort in to seats that they would have seen as safe in the past.
party activists who live in either a safe constituency, or a 'no hope' one may get asked if they would volunteer to help out in a constituency where their efforts will be more use. (in the past, labour members in - then - safe tory wokingham were asked to go and help out in one of the marginal Reading constituencies.)
there is an argument that the labour party machine in 2017 put too much effort / resources in to defensive marginal seats, and a bit more effort in a few target seats might have made a difference. Although the tories misread the polls, or theresa may wouldn't have called an election in the first place.
broadly speaking, all three main parties will put candidates up in every constituency and talk about winning, but some in safe seats will know they don't stand a chance, and it can be a training exercise for newcomers in the hope they will be given a seat where they actually stand a chance next time (tony blair's first election was being stood as a candidate in tory buckinghamshire, and one of the current tory 'elder statesmen' stood against dianne abbott in hackney as his first go.)
although it's not the done thing to say out loud that they know they won't win and hope to get a better constituency next time - labour candidate a few elections back in wokingham (then seen as very safe tory seat) got caught saying that and had to be replaced in a bit of a hurry...