Jeremy Clarkson is not talking literally. All of the underlined passage is figurative, and is instantly recognisable and amusing to British men of a certain age....
So, being British, the engineers took their pipes to the potting shed and realised that the propeller's tips were encountering Mach problems."
British men are said to love our garden sheds [potting sheds] as somewhere to get away from our wives; either by ourselves, or with like-minded friends; somewhere to "potter about", doing things that we want to [as opposed to doing things that our wives tell us to]. Also, the great fictional detective Sherlock Holmes smoked his pipe whenever he wanted to think his way through a particularly difficult puzzle ["This is a two pipe problem, Watson", he said...]. So, Clarkson is being whimsical and amusing by playing on British stereotypes. He is not actually suggesting that the Mach problem was solved in a potting shed, as opposed to a wind tunnel or laboratory.
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