I’ve found Branston Pickle, as well as Marmite, at Wegmans, the old A&P, ShopRite and Stop and Shop in the int’l section. It’s pretty common in stores now. Whether it’s popular in the US, I don’t know. I happen to really like it.
We have a ShopRite, and I will look for it. It’s interesting that people seem to especially like it with cheese sandwiches.
-JT