Could they not all register for a proxy vote? According to the DDC website, this can remain open indefinitely for future elections as well. For anybody whose work can be expected to see them out of the UK this would seem to be the ideal option. If so, I am surprised that members of the armed forces are not encouraged to register for proxy voting when they join up.
http://www.dover.gov.uk/council__democracy/elections/voting_by_proxy.aspx
As someone who was deepsea in the Merchant Navy for over twenty years, I must have missed out on voting several times for the same reason. There was certainly no option for postal voting, it would have delayed the election results for up to several months waiting for merchant ships to get into ports around the world with any sort of postal service and then awaiting the returns.
I recall that when I spent a year on Israeli ships, they tried to do exactly that. Even with their small merchant fleet, it delayed the results of the election by many weeks and I think it was the only time they tried it.
For units of H.M. armed forces such as warships, air bases, barracks, etc, I would have thought it would be easy to register each one as a polling station, appoint an officer to count the votes, and email the results over the Satcom. This would probably also cover most of those engaged in active operations as there is several weeks notice of an election.
Information for members of the armed forces to vote by proxy here:
http://www.aboutmyvote.co.uk/register_to_vote/armed_forces.aspx