ok - I've found this:
"The other day I watched part of a low-budget hastily-cobbled-together TV programme about how the Industrial Revolution stimulated the development of military hardware (or possibly vice versa). There was a sequence in which a soldier in the Napoleonic wars stuck a loaf of bread on his bayonet, put his helmet on top and poked it out from behind a tree, so that a French sniper fired at it, was thus located, and shot dead: well done, that fellow with the bread!
The telly pundit who was presenting the programme waved his arms about a bit, gave a sly grin, and said “And that’s where we get the expression ‘to use one’s loaf’”!
“That’s interesting”, I thought, “I never knew that”!
Then I thought, “Hang on, that’s rubbish: ‘Use your loaf’ must be rhyming slang, loaf of bread, head…..oh, for God’s sake.”
So what was that about? Did the presenter really believe what he said, or was it a heavy-handed spoof? The sequence was irrelevant to the subject of the programme, so either way it was fatuous."