What happens after an eye test?
Following an eye test your ophthalmic practitioner is legally required to give you your optical prescription or a statement to say you've been referred for further tests.
An NHS optical voucher will also be issued immediately if you can prove you're entitled to one. There are currently 10 voucher values for glasses and lenses. The values depend on the strength of your prescription. The stronger your prescription, the higher the value of your voucher.
You should never feel obliged to buy glasses or redeem an optical voucher from the place where you had your eye test. Shop around for the best value and only buy glasses or contact lenses when you're happy with the product and cost.
https://www.nhs.uk/nhs-services/opticians/visiting-an-optician/