Gluten Free Restaurants in London

You don't say where you're from, but you'll generally find every major city in Europe/UK to have many more restaurants that know more about GF food than anywhere in the US. Yes, there will be exceptions but I stopped worrying about finding GF food in Europe/UK years ago. I was diagnosed with Celiac in 2004, the dark ages for awareness in the US. The following year I went to Ireland and even the pubs in the smallest towns knew what Celiac was. In 2018 in Manchester, I randomly went in a restaurant that had the most amazing GF fish and chips. I also had GF fish and chips in London a couple years earlier but I did seek it out that time-just google, I can't recall the name of the place but its well known. If you like Indian, I highly recommend Dishoom.
Seriously there are a ton of resources online but really it's not even necessary. You'll eat well!

Awesome, from the US and thanks!

I've found 2 apps that have been extremely helpful, "find me gluten free" to review restaurants and "gf scanner" for grocery shopping. There are of course restaurants that haven't been reviewed yet or items in our local grocery stores that haven't been reviewed but overall both apps have made life easier for us.
For restaurants using the map, I was even able to search an arrondisement in Paris so I know ahead of time where it would be best for me to go. I haven't used the apps in Europe yet since last fall I didn't know I had celiac. Hope this makes your life easier too!

By law restaurants in the UK are required to list all allergens on menus including gluten, this may or may not have increased awareness about coeliac disease but I can't recall the last eatery that I visited that didn't offer gluten free options. You won't need to use any resources to find GF options as you're pretty much guaranteed to find GF everywhere you go.