I feel qualified to speak on this subject as once years ago I worked in Quality Control at United Biscuits which made KP Crisps. It has since closed down (draw any conclusions you want) and I haven't seen KP Crisps around for a long time - it seems to be all Walkers nowadays.
However, when I worked there on the crisp line after they had done the KP they then did the Marks and Spencers on exactly the same line, the same crisps, except that they were in different packets, and of course the Marks and Spencer would have cost a lot more. So the only difference was the label - it was the same product. I bet if you gave someone a packet of KP and a packet of M&S and asked them which tasted better I bet they'd go for the latter. A sort of placebo effect, except with crisps.
I myself only buy own brand products as a result of this crucial insider knowledge. As we normally shop at Morrisons the own brand will be considerably cheaper than the branded products sitting to them, and could well have higher production standards, as the factories will want to keep the supermarket order.