I found a natural foods store in London!! It was right near where I lived for 3 weeks in 2007, near the Russell Square stop on the tube. I don't think it was there then, although I wasn't really health-conscious then so it's quite possible that I just never noticed it. Prices on their own seemed reasonable; however, with the dollar being so much lower in value than the pound I didn't buy anything there. Good selection and assortment though, especially for a relatively small store. They had a cold/hot salad bar too with lots of yummy lookin things.
I ate a lot of salad before my flight on Tuesday but haven't been too raw since... However, I am looking forward to being in France tomorrow and soaking in all the fresh, wonderful, delicious produce!! More to come soon :)