British Expats living overseas have the right to vote in all National and Local elections in the UK. However you need to register in order to qualify. Providing you were registered to vote in the UK during the last 15 years the process is easy and applications can be made online.
For people who have been away longer the process is slightly more complex and this covers young people now old enough to vote.
Full information on your rights can be found under the article British Citizens living Abroad on the About My Vote web site.
Since one of the options offered is postal voting early registration is important with a general Election due next year.