Veterans UK, part of the MOD, have now extended the availability of the 'HM Armed Forces Veteran Card' to all former members of the UK regular and reserve forces:
An HM Armed Forces Veteran Card is a way to prove that you served in the UK armed forces. The card can make it quicker and easier to apply for support as a veteran.
It’s free to apply.
You can currently only apply for a Veteran Card if you have a UK address. Veterans who do not have a UK address will be able to apply later this year.
Veterans can apply online or by post. The card has been automatically available for some time to new leavers.
This link explains everything you need: