I'd check with the airline.
It depends on which terminal you'll be in as to where you eat but the UK have pretty strict guidelines about labelling, places like pizza express and pizza hut have allergy labels on their menus. Otherwise you could get takeaway food, sandwiches, pasta salads etc from places like Boots and WHSmith. I'd be wary of coffee places as they may claim to be nut free but often have things out in the open and serve almond croissants next to regular ones.