Not a lot at all.
Yorkshire Tea.
Twinings lemon and Ginger tea.
Shredded Wheat.
Loyd Grossman pasta and curry sauces.
I will always try own brands and if I like them, I keep buying them, If not I don't bother and try something else.
I find certain products like the pasta sauces vary a great deal in flavours I enjoy compared with other brands.
With other stuff such as toilet rolls I go for supermarkets own top of the range, so as not to waste money. Just because there is a well known brand name on it does not mean the product is essentially better than a similar product.
The classic example is generic over the counter medicines. Exactly the same ingriedients and about a quarter of the price of branded goods.
Aldi dishwasher tablets win "Which" magazine top awards and are lots cheaper than Fairy products.
It seems to me that our choice of brands of Tea and Coffee have a much deeper loyalty than most other food products.
I would suggest that many top price own brands of say standard tinned fruit and veg goods would be indistinguishable in any properly conducted double blind surveys.