I just saw this today and I'm a little excited! Amazon are selling groceries now. The website is not the greatest but if you type in the right word you can find a whole list of American items. The prices are moderate to high but for example a box of cereal is £6.00 as opposed to the usual £8.00. They don't have a huge selection but it's a good start!

Here are the highlights I found:

Double Stuffed Oreos - £5.47
Fruity Pebbles - £6.47
Mac n Cheese - £11.88 (for 12 boxes)
Aunt Jemima Corn Bread - £3.47
A&W Root Beer - £11.97 (for 12 cans)

Do I really need a box Lucky Charms? Maybe.