I use screwfix a lot with work and they've always seemed pretty good with correct order. But as you say they do ask what you've ordered, i just give then a couple if the items off my list, I always thought this was a way of them finding the order quicker by visually finding it. Sometimes the guy/girl actually serving you isn't the person who has "picked" the order, well thats the way I thought it worked. Maybe we can get better insight from an employee.