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.