I think "not doing anything to it" includes repackaging. I think this was a problem for Irish supermarkets, that items were being imported from EU to UK distribution centres, where they were put into Irish boxing and sent across the Irish Sea. This meant they were re-imports and subject to duty. If they just passed through the UK and never left the lorry, they would only be in transit, no duty. If they were substantially altered, they would be "cumulated", new product, no duty.
Not sure I haven't mixed something up there...