How to save £150 a room on underlay

When getting carpet for my house not knowing where to go I went to a Carpetright superstore. After they quoted I noticed that the luxury underlay they had convinced me was worth upgrading to was bumping up the overall price significantly. After shopping around I found what appears to be a very similar product at  Screwfix but at less than half the Carpetright price! See screenshots from the company websites below:

At £5.69/sqm cheaper on a 26 square meter room you would save £147.94! I also found Screwfix significantly cheaper for the other carpet extras, grippers, door bars etc.