Traveling to the UK

Look up Holland & Barrett as there is bound to be one nearby. It's a health food store where you can pick up healthy snacks and protein shakes etc.

I love Wok to Walk where they make stir frys fresh to order made with your choice of ingredients. They have a branch in Soho which I've not been to but imagine it's just the same as branches I've been to.

For basics there are supermarkets everywhere, smaller branches of Sainsburys (Sainsburys Local) and Tesco (Tesco Express) where you can pick up essentials and pre-packed sandwiches etc.

Higher end supermarkets include Marks & Spencer and Waitrose where you will get good sandwiches etc. at a slightly higher price.

If you eat at chain resturants most will have nutritional info available online.

jump on a tube, go to Brick Lane (the East End) for a curry

go to Edgeware Road (Hyde Park and Marble arch join the bottom of this) got to Maroush (they all seem to have this name so give one a try)

Bella Italia is a good price with a good menu but probably carb heavy. Have the arabiata.

You will also be quite close to China town you can get nice food there. You could walk there easily.

But also there are a lot of chains that do mexican etc so you can have high protein low card options. I love Nando's when I am on the go.

Enjoy London

Further ideas:

Branches of Waitrose sell good quality food similar or better to Marks and Spencer or Sainsbury. You'll find diet options easily available. There are branches across London.
http://www.waitrose.com/content/waitrose/en/bf_home/bf.html

A West End of London institution is The Stock Pot - if London food prices are hurting you should find a good meal at a reasonable price. Its not five star, just good value.
http://www.stockpotlondon.co.uk

Another chain for a quick spicy meal is Wagamama which started in London and has gone global
http://www.wagamama.com/restaurants

Just because you're dieting doesn't mean you shouldn't have some fun at the original Hard Rock Cafe. They serve salads!
http://www.hardrock.com/locations/cafes3/cafe.aspx?LocationID=91&MIBEnumID=3

Enjoy!

Brick Lane is an interesting experience, but some say you can get better food off the strip.

If you are willing to make the trek to East London, two iconic places for Punjabi food are Tayyabs http://www.tayyabs.co.uk/ and Lahore http://www.tayyabs.co.uk/, both close to Aldgate. Expect to queue for a table at either place if you don't go early or reserve. To control calories I would go for the Tikka, salad and plain rice. The grills are amazing at either place. The aubergine and lentil dish at Tayyabs is great too. Very cheap by London standards. You can bring your own wine to Lahore as they are unlicensed. Tayyabs too probably, but I am not entirely sure.