Screwfix when it first started was the the go to place for screws but these days the range has diminished. Your OK on the larger sizes and lengths but getting hold of smaller screws below 25mm in length is hard with very little choice. Round here we do have alternative suppliers but not as convenient as popping down to SF.
Another gripe is the accompanying pictures often bear no resemblance to the actual product ie back to screws often it is just a generic picture. As for their sizing well it can mean anything. I bought a Y shaped hook, in the catalogue it?s size was stated as 25mm., turns out that the only thing 25mm was the length of the screw not the important measures like the length or distance apart of the prongs.
Richard