Its normal for any trade to put a mark up on materials, as mentioned, they have to buy it, fetch it, store it, sort invoice etc, but that said, £65 for 100m 2.5 is steep! Even
wickes is flogging it around £33 at the mo.
FWIW, i really dislike customers buying stuff like cable/cu/accessory etc. (Only exception is decorative fittings) They will buy the cheapest rubbish possible and then expect the electrician to deal with it. A good spark will give a warranty on his work, he wants to have a good idea that the stuff supplied isn't going to drop to bits in six months.
Although it could happen with any supplier, one of my pet hates is t+e where the sheath is stuck to the cores, a real PIA to strip. At least if i buy it from my preferred supplier, if its crap i just take it straight back, moan at them, get another. No receipts needed, no time spent at the 'customer service/returns desk' talking to a spotty teenager dope filling out credit notes etc
You dont need to see registration paperwork, (i never carry my ID) just ask him for his trading name, then find him on the competent person register.
Have you yet found a sprky who will do something about your inaccessible JB's?
If you have and his price is middle ground, he might be a good choice.
Don't forget that the price to swap is partly about the materials and labour for the swap and adding bonding as required, but the spark should spent some time testing the existing circuits before reconnecting them. Its not just about whats in front of your eyes.