Was thinking about this this morning.
We live in a very safe Lib Dem seat. I wouldn't say I'm a natural Lib Dem voter but can see the MP does his bit.
However, if we did want to vote Labour or Tory, it would be wasted. There is never much of an opposition here so they wouldn't tally enough votes and once the Lib Dem MP got in, that would count as one seat at Westminster.
Why does it work this way? Though I am british (having been born here), I spent many years living abroad where they had the every vote counts system. I'm not entirely sure how it worked (must look it up!), but let's say you were a die hard Tory and just happened to live in a safe Labour seat, even though your vote wouldn't influence the outcome of your MP, it would go to count towards a national total.
I think the voting system here contributes to voter apathy.