The voting rights of expats has been considered a contentious issue for many years. There was previous confusion as to the actual rules, but the electoral commission has codified your rights.
You may vote in British elections if:
1) You have been registered to vote in the UK at any time in the past 15 years and
2) You have been living abroad for less than 15 years.
There are an estimated 5.5 million British citizens living abroad, yet very few are registered to vote. In fact, just 20,000 expats were registered in 2005, according to the Conservative Party website.
There are two ways to vote if you are living offshore; via a postal vote or by use of a proxy. A postal vote is the easier option but there are always the potential problems with delivery, delays and temporary absence from your residence. Voting papers must be received in the UK by the day of an election.
Voting by proxy is a more realistic option if you wish to make sure your vote counts. This means you nominate a UK resident to vote on your behalf. In most cases this will be a trusted friend or family member. Registration forms can be obtained from the major parties websites.