Together with Another Europe is Possible and the3million, British in Europe are campaigning for all UK residents and UK citizens living abroad to have the right to vote. The 3 million recently published this information to all its members about registering to vote in the UK.
The campaign website, where you can sign a petition and contact your MP is at: http://www.letusvote.org.uk
You can also sign a UK Gov and Parliament Petition to “Allow all British citizens to vote should there be a new referendum on Brexit” here
British in Europe and the3million are also partnering with Best for Britain in a campaign for all UK residents and UK citizens living abroad to have the vote in any second referendum. You can find out more about their campaign here.