In areas that are considered to be "safe", a) voters are less likely to be interested and b) parties are less likely to run competetive campaigns, targetting resources and activists on marginal seats they may gain or lose.
Electoral Reform Society: Election already over in nearly 400 seats:
The Society has listed 382 seats which are ‘Super Safe’ in that they will not change hands even with a landslide on any conceivable scale. The Society points out, however, that there are many more seats where the outcome is a very safe bet, even if an upset is not beyond probability.It is my belief that turnout is likely to go up, overall, in this election as it's the first election since 1992 where the overall result is not a foregone conclusion.
But for residents of 382 seats out of 650, the local result is already a foregone conclusion. There's a spreadsheet on the site to download; if you live in one of the seats listed, and you're not sure you want to vote, make sure you're registered to vote. Go to the polling station.
Don't put an X in the box.
Write "No Safe Seats; make my vote count" on the ballot paper.
Why should you do this? Because at an election, the returning officer must get the agreement of a representative of each candidate before a ballot can be rejected. Your already selected future MP will get to know how frustrated you are.
Prediction: after the election, if it's as close as it is now, a large number of Conservatives will complain that they were robbed and that Labour got more seats than they deserved, or words to that effect; you already see this with the "we won the votes in England" meme. What they don't take into account is that the 'safe' Labour seats are very very safe. Turnout is incredibly low in many of them; that doesn't necessarily indicate disaffection, it just indicates that there's no point in going to the polling station when you know the MPs won already. Labour seats see a much stronger falloff in turnout than Conservative seats, Lib Dem seats are in the middle.
The Conservative party says they like the voting system as is, rotten boroughs, safe seats, differential turnout and all.
It's a damn shame that they've never bothered to try and understand it.