If you are not on the electoral register, or if you are but want to get a postal vote, the deadline for geting the forms in is this coming Wednesday (31st).
You don't need a special reason to get a postal vote, you just need to fill in a form to ask for one.
You can get forms from your local council, which in the case of Merseyside will be Knowsley, Liverpool, St Helens, Sefton or Wirral.
But you can also download the forms from the electoral commission and then send them back to your Council.
You'll find the registration form at this link and the postal vote application form here