British citizens living abroad allowed to vote:

Overseas electors

• can vote at UK Parliamentary elections after leaving the UK there is no time limit on when they left
• overseas electors can be registered either 
  • based on their last registered address in the UK, or if they were not previously registered
  • the last address where they were resident in the UK
• cannot vote at local government elections or Police and Crime Commissioner elections

Further information on voting when abroad on GOV.UK's website

Service voters and crown servants stationed overseas 

• can vote in all elections

Armed Forces (Army, Royal Navy or RAF) posted abroad and their spouses or civil partners

• can register to vote through making a service declaration, which is available to download from the Commission’s website. (Service declaration forms should be sent to the elections office for the area where you wish to register) 
• can register at the address in the UK where you would be living if not posted overseas or at an address where you have lived in the past

Armed forces (Army, Royal Navy or RAF) living in the UK and their spouses or civil partners 

• can register to vote through making a service declaration
• can opt to register to vote in person at their polling station or by by post or proxy

Crown servants, UK embassy, European Union or United Nations staff based overseas and their spouses/civil partners

• can register as a Crown servant, application available to download from the Commission’s website or you can complete an online application
• cannot register if you are employed by the EU, UN or any other organisation not directly in service of the Crown

Further information on overseas, service voters and Crown servants