Hi, when shopping with my Australian fiance for Nappy San (don't ask!!), we discovered you can also buy North American food (not just Australian stuff!) at this place: Australia Shop, 27 Maiden Lane, Covent Garden.
Last time I was there, they had the Kraft Macaroni and Cheese kits, etc. It's mostly geared towards Canadians, but a lot of food you can buy in Canada you can buy in the USA, etc.
The shop is located across the street from a cool bar called The Porterhouse, and it's next door to La Perla, a pretty decent Mexican restaurant (though nothing compared to the real stuff back in Texas!) so you could spend a whole evening in the area!