I found a box at the Tesco near Covent Garden. Just like how I remembered them!

DH wasn't impressed, but that's okay. Like root beer, more for me!
Surprisingly, they weren't in the American or international section; they were in the aisle with the usual crackers and crisps. Though, heads up to anyone going to get some, a box was something around £3.75. Yeah, almost 4 quid. Our fresh red velvet cake from the bakery was cheaper than that...