Grim, the secret is to think like a civilian and not a serviceman.
Food could be tricky. Should you try to claim for mess food, which is cheap by open market standards, then you might face additional tax for benefit in kind.
OTOH if you have an open market meal on duty, in a hotel say, and the cost is more than your subsistence allowance then you can claim the difference.
Accommodation I think would be treated the same way should you me in SSSA and pay more than the allowance.
Another claim, often overlooked, is the cost of uniforms. An officer's salary reduction for uniform maintenance covers the replacement of all his working dress over a 5 year cycle; it does not cover extraordinary expenditure. That might be the Bates Cap, Church shoes, Dent's glove, a new No 1, 5 and 6. Keep receipts and claim.
If you are going outside Europe you can even save VAT if you have stuff posted to you at your overseas unit.