Will be traveling to London in May with my son is has severe allergies to peanuts, tree nuts and shellfish. I've already read that the UK is good with allergies and restaurants and will ask. I'm specifically looking for names of places if anyone has traveled with a FA child.
No place specific, but will just confirm that legally they are obligated to provide allergen information on 14 allergens for all menu items, and in every restaurant I have been (sit down type place) the server has asked about allergies. I do suppose there are some that may not go so far as to guarantee no cross-contamination, but I have been impressed to the extent they make accommodation or provide guidance.
Chains like Nandos and Wagamama I have seen high praise for their efforts in menu labeling and food handling, so those two might be options (and pretty good food as well)
An important edit: Since you mentioned Shellfish as an allergy, be aware that in the UK and most European countries, they distinguish between Crustaceans, things like shrimp, lobster, and crab; and Mollusks, things like clams, mussels, oysters, etc. In the US we tend to lump it all together, in fact you were likely only told the child is allergic to "Shellfish". The point being, read through their descriptions of allergens so you can "speak their language" so to speak. Even though both countries speak English, it is not always the same language. Similar with Peanuts vs tree nuts, but that distinction is more common in the US.
I traveled to England with a significant dairy allergy. After about a day I was ready to move there. It's not like the US at all ( if that's where you are from.) Everything is very clear, allergens are clearly labeled, and in some cases they bring out a tablet, go through your allergies and then have you sign it! I did not deal with cross- contamination issues once. Even in a non- touristy town I visited for a conference was amazing with allergies. I hope you have the same experience I did.
I have a friend who does not want any black pepper in her food so she always tells the server she has an allergy to it. Like the others have said, they are very accommodating and conscientious about food allergies. Once we were at a Dishoom and the server even brought out a binder with the menu items and specific ingredient lists for each item. I was impressed with how thorough they were.
I can't speak to those allergies specifically but I was traveling with someone with a wheat and gluten allergy and it seemed that servers were so much more aware than in the US. The first question was always "are there any food allergies?".