I've seen black beans at Borough Market. But you can always buy the canned black beans, which I've seen at Sainsbury's in Angel Islington.
I've never noticed a difference between UK or US Hellman's Mayonnaise.
BUT there is definately a taste difference between U.S. Coke and Coke in the UK, or anywhere else in the world for that matter. The reason is that US Coke is made with corn syrup; international coke is made with sugar. I much prefer the international sugar coke, which is actually slightly less-sweet than US coke.
The other thing here that's strange are El Paso Mexican food products. Nothing tastes the same to me, especially their corn tortillas (which are actually only 40% corn and 60% flour). The enchilada sauces all taste different too. This stuff here is made in BELGIUM! What do the Belgians know about Mexican food?
)
The other thing that's hard to find here is French's American yellow mustard (they do stock it now in our local Sainsburys). Most of the mayo here is spicy English mustard I can't stand. Hamburger Union only serves their burgers with French's. Woo hoo! (Incidentally, this is the absolute best burger I've had in London).