To be honest, Parties have a very good idea of how any individual will vote.
They use profiling software that is address specific and will tell them your buying habits, the kind of car you probably have, the newspaper you probably read, the kind of job and educational level you will have. From this it is very, very easy to predict your likely voting preference.
Various groups are also more likely to vote - older people for example, or various social-economic groups. So effort is concentrated on those sectors.
Knocking up tends to be done only in marginal wards in local elections. When I was a councillor, I didn't campaign in my ward because it was a safe seat, I campaigned, did telling and data entry in a neighbouring ward that was a very tight marginal and every vote counted.
Parties are not only looking to see if their own vote is out, they are also looking to see if the other party's vote is out.
In General Elections, volunteers from other areas get sent down to marginals to mobilise the vote and no-one bothers sending people in in safe seats.
So, if you live in a street that profiles you as an almost dead-cert labour voter in a Tory/LD marginal then you are likely to be seen as a prime target as a squeeze voter (ie since your likely political party doesn't stand a chance then you could be persuaded to back the lesser of two evils rather than waste a vote).
On the whole staunch Tories are not a good bet for changing their allegiances for tactical purposes - unless you are from UKIP - and so the LD/Labour teams will keep away.
For months before any election, computer data will have been profiled and lists drawn up of defs and probs (definites and probables) and the no chancers or known members of other parties.
It is quite frightening how much information is out there on everyone - my parents didn't believe it until I printed off the 8 pages on their address and likely profile! It was scarily accurate.