Free eye tests

I am short sighted and I think I need new glasses, as my vision is not as crisp through the glasses as it was before, especially at night.

I last had an eye test 12 months ago.

I know I am entitled to an NHS eye test due to my benefits but this is every 2 years.

I explored the possibility of paying for an eye test with the intention of having my NHS eye test every 2 years and paying for an eye test every other year, so I have an eye test each year.

However unfortunately I discovered that if I pay for an eye test this resets my 2 year timer for an NHS eye test so I have to wait 2 years from the date I had my private eye test.

However…I read that I can in fact have an NHS eye test early if my optician deems it necessary. I am just wondering how I go about getting this? Do I need to have an eye condition, rather than just short sightedness, or does natural deterioration of vision count as a valid reason?

Thank you.