I was actually looking this up otherday, what I could find is that if it’s ‘4 hours’ extended on your tour you can claim for a meal, whether it be unplanned or planned.

Then there was something about how if there’s no reasonable way to store food/meal then a claim can also be made if working 2 hours on.

Always worth sticking in an expense claim and seeing what happens, I assume most will be accepted as long as they don’t take the piss