There is a store in Bluewater, f you live anywhere near there, that sells many American brands but they are expensive. For instance a friend asked us to bring back some Lucky Charms breakfast cereal for her son of 22 (he has Aspergers). We paid $5 for a 2kg box in Walmart but in Bluewater she pays £7.5 for a 750g box.
They have many brands. Some things you can pick up in Asda etc, such as Betty Crocker cake mixes, Oreo cookies etc, to make it up.