If you are a British citizen living outside the UK but you were resident and registered as an elector in the UK during the past 15 years, you can apply to be registered as an overseas elector.
In order to do this you will need to fill out the overseas elector application form and return it to the electoral office. You can find the application form online.
People employed by the government or by the British Council to work overseas for considerable periods can apply to register as Crown Servants. For more information see Overseas voters.
Note, however, that overseas electors cannot vote at local elections.