I forgot one of the most obvious, cheese strings. Very handy to carry, as long as you remember to take any leftovers out and put them in the fridge when you get home.

Most sandwich/bagel places will do a "chicken salad sandwich, hold the bread" and give you a small chicken salad on a plate.

Down to Earth in South Great Georges Street do Go Lower bars. They are low carb nut and seed bars and don't contain sugar alcohols. Mind you, at E2.05 for a small bar, I'd keep them as an occasional treat.

The Asian Market in Drury Street has jars of coconut oil for E1.65, which can be used to make all sorts of low carb goodies.