If you want to be able to vote in the EU Referendum on June 23 you can register to vote in the UK by postal vote. This will also allow you to be able to vote in General Elections.
This is the link to be able to register online. https://www.gov.uk/register-to-vote
You can register as an overseas voter for up to 15 years after leaving the UK, as long as:
- you’re a British citizen
- you were registered to vote in the UK within the previous 15 years.
You can also appoint a person to vote for you by proxy. It very likely that by the time your ballot paper reaches you here in Greece and you post it back , that it will be too late. You need to complete a form to do this and this is the link. https://www.gov.uk/apply-vote-proxy
This slightly out of date article prompted me to look into the above.