I have now relocated to the other side of the world, literally and now reside in New Zealand. This doesn't help anyone looking for a cheap feed in London, but i can continue to update my lunches and see how they compare.
The best value place i have found over here for lunch is the humble supermarket. But New World Thorndon, so fresh made to order sandwiches, just like home.
To order a sandwich, you grab your highlighter and one of their pre printed pads with your options and start highligthing until you have created a monster sandwich.
Being a cheapskate i get two, but the cheapest possible sandwich, that costs $3 each and can have only one salad item. Fortunately for me, condiments don't count as an item so i get loads of bits to make it taste nice.
I took a pic of the giagantic filling on my egg sandwich (don't forget the mayo, which over here is the same as salad cream)
The egg component dwarfs the bread, i think my my guess the egg layer is about 2 inches thick, thats a lot of egg.
I can't actually find anything better at the moment. There is a chippie down the road and a wholly bagels, but i wasn't too impressed with the pizza, it said it had toppings, but i only found cheese and tomato on a piece of soggy cardboard.