I assume you mean 'efficiency' in terms of output in W/m2?
I don't know, but I have some ideas:
1. Testing is being performed under traditional British test conditions, not EN442. The EU standard is now Delta T 50, which is now also the UK standard. In short, this means that the average radiator temperature is 70°C (flow at 80, return at 60) and the room temperature 20°C for the purposes of testing, so the difference is 50°, and that the radiator is plumbed in with connections top and bottom at the same end. Ancient British method assumed a 60° difference under test conditions, which makes the result far different. Additionally, plumbing it in different configurations such as Bottom Opposite End and Top Bottom Opposite End makes subtle differences to output.
2. Imported radiators with made-up standards of testing.
3. Wickes radiators genuinely have higher outputs per surface area.
Generally the higher prices radiators are better finished and have better paint finish, less visible spot welds and nicer detailing. The cheaper ones can be lighter, so thinner steel (may last not so long if good and sufficient inhibitor is not used), but will still heat your room. Fitted cheap Screwfix rads in my mother's house in around 2009 which worked well at least until after she sold the house three years later, though they didn't feel very solid.
