My understanding is that there are 3 different grades of progressive lenses, with more or less width to the higher magnification part. The narrow ones are cheaper (although I can't see why it would cost less to make a lens one shape instead of another). But the narrow ones take a *lot* of getting used to (up to 6 weeks of nausea, disorientation, and headaches, although only 2 weeks for me), and some people don't ever get used to them.

Specsavers is more a marketing/sales company than an optician. They advertise cheap prices to get customers in the door, with the intention of up-selling them from the cheap and nasty item they advertise to the expensive and adequate item the customer needs. If you don't like the cheap ones, 2-6 weeks later Specsavers will up-sell you to the expensive ones. Needless to say, I prefer to go to an actual optician.