The Votes for Life Bill is to allow UK citizens living outside the UK to vote in parliamentary and EU elections, even if they have lived abroad for more than 15 years. The government currently intends that even if this bill is passed before the EU referendum, it should not apply to that referendum.
There is a petition for UK citizens to sign if they want to be able to vote in the referendum. Here is the link:
The petition must have been started in October as it ends in April. Apparently there are about 2 million UK citizens living abroad, but some of them have been abroad for less than 15 years so they are entitled to vote here.
Some sources: Votes for Expat Brits blog
a BBC Radio 4 podcast: Carolyn Quinn explores the practical process by which Britain would exit the EU if UK voters opt to leave, and looks at the experience of Greenland, which quit the EEC in 1985.