Yes, you can get diet soda everywhere
London is fantastic for eating these days - definitely, definitely look at the Time Out website!!
Ceviche, Koya, Song Que / Viet Grill / Cay Tre, Brasserie Zedel, Orchard - all worth a quick google. And many more!
I just mentioned on another thread that Itsu and Leon are both good, healthy chains for food on the go (not fine dining, but a good quick lunch if you're shopping) - I love Leon's superfood salad. Both list full nutrition info on their websites.
There are some good healthy frozen yoghurt places in London - if you google Snog or Frae, they both have a few branches. There are more but I can't remember their names!