I’ve put in my application for a postal vote this morning. It’s very easy to do if you know your NI number and you’re asking for a one-off postal vote form to be sent to your home address, and your home address is the address on the electoral roll. You don’t need to scan your passport or driving licence although you do need to scan your signature. Easy enough these days as we all have decent cameras on our phones.
https://www.gov.uk/apply-postal-vote
(Edit) Should add, this is especially useful for Scottish voters as many of us will be on holiday in the first week of July. The process for applying for a postal vote for Holyrood is still a bit old fashioned and involves downloading and posting forms, but Scottish voters looking for a one-off postal vote in a Westminster election can use the gov.uk fully online service.