Dave, expats can definitely vote, most countries allow for it. Italy even has Senators specifically reserved for overseas voters to give them their own constituency.
Friend of mine (well, friend of a friend that I've met a few times) was chair of the UK&EU Obama campaign, contacting US expats to make sure they registered and voted.
Ghost, last I looked (ten years ago, might've changed), you could chose where you voted. The Tories used to be notorious for getting expats to register in marginals, sometimes that they had no links to at all, because expats, tending to being wealthier, also tend to vote Tory. I suspect that's been changed but don't know.
CG, nice contributions, and I today share your pain; I used to work in Westminster (briefly) for the Lib Dems, and I'm still involved in my local party (chair of my branch).
The observation made by someone up thread about marginal/safe seats is very true, I now live in a three-way marginal where the sitting (Labour) MP is standing down, so every vote here does count. But I come from a seat where you could put up a monkey in a suit with a blue rosette and it'd come in.
I'm actually in the middle of researching/writing an article about turnout and safe seats--this "apathy" meme going around is a myth without actual substantiation, in safe seats the parties don't campaign and the voters don't vote, in contested seats the parties campaign and the voters do vote.
To the Australian who asked why the Lib Dems don't do well? 25% of the vote last May in the local elections, 20% plus in the most recent few General Elections. Remember that you have a preferential voting system, in the UK you have to guess at the best place to put your X, and because most people think the Lib Dems can't win, they won't vote for them. Self perpetuating circle unfortunately.
FWIW, when I lived in Exeter I voted Labour to keep the (then) bastard Tory candidate out (CG may know the name Adrian Rogers, he got expelled for being a bit too extreme). Now, if I lived in a Tory/Labour marginal, despite my party membership, I'd probably vote Tory, 12 years of broken promises on essential reforms have really pissed me off.
I don't think the Tories would be much better than Labour, but they'd be different, and the current lot need to be kicked out and replaced--I just hope that Dave doesn't win a super majority (unlikely given the psephology), and that we get a change to the way we count votes at some point soon, that's the best chance we have.
FWIW, on the Facebook PHPdip app, one of my 'friends' on there with the app installed is a Lib Dem MP, she played (and convincingly won) one game on there, which is cool. She was also one of the MPs leading the charge on the whole freedom of information stuff on expenses--MOST MPs aren't corrupt and haven't been fiddling, that's especially true of the Lib Dems, although even they're not squeeky clean (although nowhere near as bad as some, the worst has, shock horror, let his daughter live in his spare room)