They do "baseline assessment" when the child starts school at 4, carla - they measure loads of skills at that point (shapes, numbers, colours, letter recognition, fine motor skills blah blah) although I don't know how they are marked.
Then when they do SATs in Y2 their scores are compared with the baseline assessment, and if they have achieved higher scores than predicted by baseline, that shows some value added. It's a v imprecise science but better than nothing and at least allows for schools with a lower-ability intake to show what they can do.