Some good news. After I didn't hear anything back from AA for eight days apart from the automated "here's 5000 miles" email I got immediately after filing my initial complaint, I responded to that email and reiterated my request for compensation. They got back to me quite quickly and said yes, you're eligible - here's a form to fill out and send back to use; also upload receipts for meals/accommodation to be reimbursed for that too.

That's great! But the email says that restaurant receipts need to be itemized, showing exactly what was purchased and its cost. All I'm seeking reimbursement for is breakfast at Heathrow, which had a cost of around $30. However, the only receipt I have from the restaurant is the one from the card terminal, which shows just the base food charge and the gratuity. I don't even think they gave me an itemized receipt - it seems that at most places in the UK, paper receipts are pretty much a thing of the past.

Are they likely to refuse compensation for the meal? I'm not claiming accommodation for the overnight delay (I was staying with friends) or anything else - just that one meal.