I used to make contact lenses for 19 years and my specialist area was bifocals and toric lenses. In regards to glasses there are just a handful who make the lenses, the optician basically picks from a manufacturer.
The big players like Vision Express, Specsavers and Boots sign up contracts with the main providers, they promise the manufacturer that they will buy solely from them, then each time the contract is up for renewal the manufacturer has to basically bend over and get shafted as the optical chain force the price down further. I think Specsavers actually make their lenses in house now, they are also allowed to use the Pentax stamp as they are made to the required standard.
Now it won’t be much different but with contact lenses the manufacturer has tolerances, our largest purchaser was Boots, we had the contract for something like 12 years and the only way we made money was to be very lenient when it came to lenses being very close to the maximum allowed tolerance.
Large optician chains don’t check everything that comes through, they only tend to check things when there is a problem reported by the consumer. Smaller independent opticians can be a royal pain which is good for the consumer but not the manufacturer.
I remember Moorfields Eye hospital and an Optician named David Evershed Martin, my god were they strict, no such thing as a tolerance with them, it was spot on or take their business elsewhere. We had a hit list of opticians that we used to double check everything before the lenses left the factory.
In regards to Varifocals I just can’t get on with them, usually if you suffer motion sickness or can’t play the majority of Xbox types games as they give you a head ache then you’ll suffer getting used to Vaifocals.
Cheapest ones you might as well throw money down the pan, the field of vision is so slim. Personally for me it is reading glasses for reading and distance for driving.
Anyway, sorry for the lengthy reply but if you want quality go to a smaller independent optician. I’m happy to use the specsavers and Boots of the world as I’m lucky and can tell straight away if something is wrong, I never accept the “getting used to something new excuse”.