British citizens living abroad can vote in UK Parliamentary and European Parliamentary elections, but not in local elections. British citizens living abroad for more than 15 years are not allowed to register to vote in UK elections. You can register as an overseas elector at www.gov.uk/register-to-vote, using the address where you were last registered to vote in the UK.
A member of HM Forces and their wife/husband or civil partner can register as an ordinary elector or as a service voter. You can register as a service voter by visiting www.gov.uk/register-to-vote. A service declaration must be completed, which is valid for five years. Each unit of the services has designated one member of staff to be a Unit Registration Officer who will be able to provide further advice.