While an optometrist is someone who can undertake an eye test, and prescribe glasses or contact lenses, an optician is the person (or shop) that can make and sell those glasses.
Almost all UK optometrist’s not only prescribe glasses, but dispense them too (as opticians). This is because there is more profit in spectacles and contact lenses than in eye tests.
Optometrist’s (or ophthalmologist’s who are also able to prescribe glasses) are obliged to provide a prescription to someone having their eye’s tested, so that they can obtain glasses from whomever they choose.
The arrival of internet opticians who dispense glasses at around half the cost of high street opticians is having a significant impact on the market.