Any voter can apply for a postal (or proxy) vote in the usual way for May’s elections, including the AV referendum. However, people who have previously applied for a permanent postal vote may also be entitled to one without having to re-apply.
There are three categories:
1. People who have a permanent postal vote for a UK Parliamentary election – they will get a referendum postal vote too.
2. People who have a permanent postal vote for a local election and are on the register for somewhere that is holding an election in May – they will get a referendum postal vote too (if they are are entitled to vote in the referendum).
3. People who do not fall into either of the above categories but apply specifically for a postal vote for the referendum, or for both the referendum and other election(s) in their area.
If you want to apply for a postal vote you can do so via the Electoral Commission’s special website. To vote in the referendum, you also need to be on the electoral register, which there is still time to join. (See here for more about electoral registration.)