Dairy-free, Lactose-Free, Vegan, and Vegetarian are on the menu, but the same kitchen also prepares what you avoid, so cross-contamination is possible. Tell staff what you react to, and ask how your dish is prepared before you order.
Niche is a British restaurant on Rosebery Avenue that describes itself as 100% gluten free, and also free from peanuts, tree nuts, sesame and oats. The site says it was the first 100% gluten free restaurant in the UK to receive Coeliac UK accreditation. The menu marks dairy free and vegan dishes and there is a separate Low FODMAP card.