I'm very much the opposite. I have in mind what I want to pay for a certain item, butter for example, and I'll buy whatever brand is being promoted that week for my target price. As long as it's not absolute garbage of course.
Maybe it's different in the UK than in other places, but the large supermarket chains are constantly running so many promotions that you can, if very brand disloyal, always shop smart based on bogofs and alike.