I'm gluten free and was in London for 2 weeks this summer. First off, awareness is way higher than it is in the US. Every meal started with the wait staff asking about food allergies. I rarely felt left out like I do sometimes as home, lots of options at most places.
A few of my favorites, keeping in mind that we mostly add convenience food that was nearby:
Fish and chips from Hobsons or from Fishoria. I liked Fishoria just a little better but their menus are very similar, so I'd go to whichever is more convenient. They also both had gf sticky toffee pudding. Ordered it at Fishoria and thought it was a little too sweet but I was glad I had the chance to try it.
Chicken nuggets from LEON. Sounds funny but it's hard to find gluten free chicken nuggets from a restaurant in the US so I made sure to seek these out on our trip. And I was glad I did. They're a bit pricey, like most food in London, but they were so good that I got them twice.
Look for GF sandwiches as part of the meal deal at grocery stores. My favorite was a simple tomato and cheese sandwich on gf bread from M&S Simply Food in the train stations.
We made the mistake of going to Pizza Express. They have gf crusts. The most mediocre pizza I've ever had. Wanted to try Pizza Pilgrim but didn't get a chance.