People in the armed forces, and their spouses and civil partners, can register to vote as a Service Voter or an ordinary elector.
If you're based overseas, or expect to be posted abroad in the next year, register as a Service Voter. This allows you to be registered at a fixed address in the UK even if you move around. A Service Voter registration also lasts for five years so, once you're registered, you shouldn't have to worry about it whilst you're busy overseas.
If you are based in the UK and unlikely to be posted overseas or move address in the next year, register to vote as an ordinary elector.