How much you can claim depends on how long you are away for (I assume not overnight), but it's going to be worth no more than 2 quid to you. And you can only claim for expenses actually incurred - no buying a cheap sandwich and claiming for a two-course lunch. And you have to produce receipts if asked by HMRC Are you sure it's worth it? Honestly, it's cheaper to take a sandwich with you.
Re the travelling expenses - tricky. Travel by public transport is claimable but it'd be a bit difficult to argue the car trip was only for work purposes if your wife is going shopping at the same time. You strike me as the sort of guy who likes a gamble though, so you could ring HMRC and ask them.

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