Gluten free on lower budget

Coming to London for five days next month, but I was diagnosed with celiac disease since last visiting. I'm a student so I don't have the possibility to just go to any place, and I found London to be quite expensive on my past visits too, so I was wondering if anyone could shoot me some recommendations on gluten free restaurants that aren't on the priciest end. Unfortunately my schedule doesn't really allow me to prep food myself so I have to eat out as I go. Maybe there are some chains that are known for accommodating to gluten free that could be found around the city? Anything helps!!!