Used to do the same thing myself.
Can still remember the arguments with my primary school teacher.
Me: "But, please miss, We DO carry coals to Newcastle"!
The ones I remember, in particular:
Dundee. Don't recall what we took, but bales of hessian (jute) we brought back from Newport (Tay) jute mills.
Cardiff. Heinz canned stuff out. Steel to Wigan Rolling Mill back.
Barnsley Sulphate of Ammonia from t coking plants to local Fertiliser manufacturer. (Was it Peak: that name seems to ring a bell?)
Fish meal from Hull also for fertiliser.
London Don't know the cargo, cause shunter took the wagon, dad had his sleep, and I went off to explore London, alone for the day.
I think I learned more from those trips than I ever did in school.