Customs can, technically, charge tax and the value of the repair/service conducted abroad. I once sent a pair of Olympus OM-2n cameras to the best tech there is in the USA (John Hermanson), and got charged on their return, not for the value of the cameras, but their increased value by virtue of the CLA they had undergone.
But, @noromance , there can be no reason to charge VAT on entry to the country, and I assume that's what you mean since it is in sterling.