If you have lived overseas for 15 years or less then you can register to vote online in minutes.
See www.gov.uk/register-to-vote.
other useful links are:-
Apply for a proxy vote when you register.
Find your Electoral Registration office
NB: The Conservative Party manifesto commitment of Votes for Life has been enacted so in the future all British citizens living overseas will be able to vote, regardless of how long they have resided overseas. HOWEVER, if you have lived overseas for more than 15 years please do not yet attempt to register until the formal announcement of the commencement date for this measure.