Firstly, you can apply for a postal vote, providing you are eligible to vote. You will not be asked to give a reason.
As you might expect these days, you can search the web for an application form by typing - how to apply for a postal vote. This page will explain all the why's and wherefore's of voting by post and allow you to download an application form.
Once you have downloaded and completed the form it should be sent to your Electoral Registration Office which is normally located in your local Council Offices.
Alternatively, you could contact your Electoral Registration Office and get them to send you a form.
Well you will be pleased to learn that you have already completed the most difficult part of voting by post, which is actually not all that difficult.
Just before an election you will be notified that your postal vote is on the way and a week or so later you will receive your Postal Voting form. This form is very similar to the form you would fill out at a polling station.
Just follow the instructions and return the form using the pre-paid envelope.