I'm not talking about supplying every item. I usually tell the contractor to supply materials that do not envisage much choice. In the electrical trade, that will include the cables, consumer unit, breakers, trunking, conduits etc. But I as the client will supply any materials that does have much choice, i.e. the lights, switches and sockets, extractor fans etc.
I will of course do research before buying anything to ensure it meets all the regulations. All the materials will be delivered before the electrician starts work. So they may see what they're dealing with and install first fix accordingly.
I don't believe in making markup off items, they're not a shop. They will get paid well for their time and expertise, that's it.
About warranty of second fix items, they will be offered by the supplier.
Training up is an option for me and I would assume for others too, but I will not take up this route at this time. Remember it is the basic training, enough for a residential house rewire, no one's talking about advanced electrical installations like in commercial settings.