I find it depends on which shop's own brands we are talking about, and which products.
I imagine the same applies in all countries, so it doesn't matter that I don't live in the UK.
Due to allergies, I only use one brand of washing powder and body shampoo, and don't experiment with others. I have, though, managed to find a "own brand" shampoo I can use.
Foodstuffs are usually substantially cheaper if you buy the supermarket's own brand, and but for a very few products the difference in taste or in quality is so little that it hasn't prevented us buying whatever it was again.