I'm unsure about TDG (Transporation of Dangerous Goods) in UK, I can only speak from my semi-professional TDG experience in US/Canada.
To ship photographic chemical, the consigner has to have a certified TDG person to pack it & prepare a waybill. There is an exempted quantity to waive, but still need extra paperwork to label as such. Then, the courier or carrier (Post Office) must have a certified TDG person & vehicle to pick up the DG. The DG have to be transport by a labeled vehicle with certified TDG driver to the destination depot. The depot at the destination must also have a certified TDG person to receive & disptach it to the consignee. Finally, the delivery driver also should be TDG certified.
I hope this "clear up" why so many suplliers don't want to deal with small purchase of phtographic chemiclas now. TDG training is a billion dollar business world wide.