You can apply for a Voter Authority Certificate up until 26th June, though personally I'd do it sooner in case of postal delays. You'll need to already be registered to vote.
If you're not registered to vote the deadline is 18th June. You should get a reminder about voter ID when you register and be asked if you'd also like to apply for a Voter Authority Certificate then.
If you're already registered but will be away on election day you can apply for a postal vote until the 19th June and for a proxy vote until 26th June.
The rules have changed this year regarding handing in postal votes at the polling station and it is now more complicated - if you find yourself needing to to this please check the rules.
All info available on the Electoral Commission website, if anyone you know is at risk of being put off by the totally unnecessary hurdles now in place please give them a helpful nudge.