D-IFF
Yes, you would be within the rules.
Yes, you could expect to audited and challenged.
No, you do not have to produce a receipt for any single item under £5, so the auditor cannot demand that you produce one in order to keep your allowance.
It is not reasonable for the auditor to ask you to produce your greasy month- old sandwich wrappers to prove your claim. How would he know where they actually came from anyway?
Being pragmatic for a moment: Stay squeaky clean. Only claim for what you have actually spent on items that are allowed. Claims for expenditure on food must now be for "prepared food", not the individual makings of a meal. Keep a hand written, dated and signed list of every item that you buy. Show this to the auditor if he asks for it.
If you do this, the auditor cannot reasonably accuse you of having ventured outside the letter of the regulations. He may question your spirit, but that is not sufficient to justify withdrawing the payment.
I still wouldn't fancy a whole week on McGrot burgers, but each to his own!
Regards
Ginseng