Ah, the million-pound question (or the thirty-quid question). If you’re on a low income but don’t qualify for the benefits list, you can apply for an HC2 certificate (also known as a “help with health costs” form). It’s a pain to fill out, but it can get you a free test and even vouchers for glasses.
Also, some big-name chains like Specsavers or Boots occasionally run promotions for free tests during certain months. It’s like Black Friday, but for your eyeballs.
And if you’re really strapped, ask your local optician if they have a charity scheme. Some do. It’s worth a sheepish “Um, any discounts?”—what’s the worst they’ll say? “No”? You’ve survived worse.