Move away from basic items, which DIY sheds buy in container loads, and wholesalers become your main option. 250A MCCB from Screwfix? 120mm Tri-rated cable from Toolstation? It's not simply a question of availability, they won't be attempting to assist you with anything that isn't in their catalogues.
In a previous job I'd use CEF a fair bit. Where the real wholesaler won for us was that I'd look at a job (networking) and (for example) see what trunking was already installed. If (as was normal) it wasn't something I recognised, usually the local staff could identify it from photos - and get the socket boxes in even for non-stock brands.
UK should be like certain parts of the world .where many items cannot be sold to members of the public as id has to be shown .mainly gas, water and electrical stuff.
And then you go down the slippery slope to restrictive trades. While that can have some positives, it has some negatives.
Take just yesterday. I went to our rental flat as the tenant reported that the boiler wasn't working and there was water dripping from it. It was immediately obvious that water was coming from the combustion chamber, which shouldn't happen in a non-condensing boiler ...
It did fire up for me a couple of times, but I noticed the pipes weren't getting very warm. Ah, so checked the TMV that's part of the circuit with the thermal store - and when I tried adjusting it to get the return temperature up, the boiler quickly got very hot and turned off. Then I realised that the very professional "engineers" that I have to use had refitted the TMV the wrong way round after removing it while on a wild goose chase diagnosing what should have been an easy fault to identify (scaled up heat exchanger causing kettling).
So thanks to employing "fully qualified" people to work on the boiler, it's failed again (after a while) because they took stuff apart they didn't need to, didn't reset it as it had been (had they done so, the system wouldn't have worked at all), and caused condensation that's probably taken out another control board. Going to be very hard keeping calm when I phone them tomorrow.
No because they didn't have the same pricelist for everyone. Trade places would have to sell at trade rates which would be the same as other trade places and retail outlets would have to sell at retail prices and wholesalers wouldn't be able to sell to the public, it was just a fairer way of doing things, you knew what price you would pay almost before you even walked in the door.
And it was illegal for the obvious feature of allowing the manufacturer to dictate prices and thus keep them up at the expense of the public. It's arguable that the other extreme is just as bad, especially when the big supermarkets drove* prices down so hard that suppliers (farmers) had to sell below cost, but retail price maintenance was very expensive for the public.
* Some have now realised that they need to support their supply chains and are less aggressive with their purchasing.