Marks & Spencer?

Nope, M&S isn't remotely like Walmart, and they definitely don't sell batteries or photographic film. Take the advice above and go to a Tesco.

Marks & Spencer = Target (and yes, I believe some of them do have batteries)

Tescoe = Grocery store + CVS, SavOn, RiteAid, etc.

We stayed out in Greenwich and took the tube out from downtown. We stayed at a Holiday Inn by that really ugly sports complex building. There is a Sainsbury's grocery withing walking distance from the tube station that was fairly reasonably priced.