Noodles

Wagamama is a cheap, easy, quick option and they have a gluten-free menu (you can see it online too so can plan what you want - online menu). There are a couple of delicious things on the menu that are naturally gluten-free and then there are a few other things they will adapt for you. I've found the one near Leicester Square the best for the waitress really knowing what she was talking about when it came to allergies but they are all very accommodating.