I am purely diy and hobby so my take on screwfix may not apply to professionals.
For small items and consumables (screws, brackets, nuts, bolts, conduit etc) they seem about a quarter of the price of B&Q, Homebase etc - eg: a box of screws vs a bag of 25 for the same price. I have a small workshop overflowing with part packets in the hope that one day it will be just what I need to finish a job. For larger items (£25+) there may be a small saving but not so much.
I think the reason is simple - the public can easily buy larger ticket items (drills, planers, jigsaws etc) on the web. People on this forum and in the trade tend to buy tools that will provide decent service/spares etc - but a week's usage by a tradesman equates to 5 years diy use. Hence price, not quality wins.
A digression - for most of the week the sheds have to heat, staff, maintain, insure, pay rent and rates etc on a 30 - 100000 sq ft shed with only a handful of customers in the store. A hugely expensive fixed cost they can only cover by making high gross margins. Toolstation/Screwfix operate from sites approx 25% of the size with what seems like a steady flow of business throughout the week.
Much of the public judge the products on appearance, colour, impulse etc. They don't have the knowledge to easily differentiate products technically, or confidence to go into a "trade" supplier.
