Something appears to have got lost here; I'm not demanding that an airline does this, I'm asking for data points of experience, to allow me to plan my trip accordingly (e.g. if allergen menus aren't common practice out of LHR, I'll eat in the lounge - problem solved!).

I'm asking this question as I've found airlines may give you one answer through customer service, and entirely another onboard.

I appreciate the comments that answer this question, super helpful thank you.