PN
Well intended no doubt but perhaps not great advice. The issue here is whether or not something constitutes a claimable allowance in the eyes of the HMRC.
Whether something is a duty as defined by the MOD is irrelevant, as are comparisons with civvy stuff.
If you saying, claim everything, let HMRC catch you...I'm all for that (it's called an allowable error), but no point in crying when the buggers have you digging back through a mountain of paperwork several years hence and then want their money back.
As to the specific question...I don't know, so I'd call the HMRC to find out.