I read that suppliers who are members of some 'coal suppliers association' undertake not to supply neat coke - it must be viewed as too hot and dangerous - but only to supply coke when mixed with other coals.
don't know about that, but i know hardly anyone has it :/ it's not very hot at all if it's not in a forge, pretty much just goes out if you stop blowing it in fact, plus house coal cokes, so i don't think it will be about heat out put, coke is just a part of bituminous coal, more likely that they only sell it with stuff added so that it will burn with out a blowen air supply.
as for charcoal, i've used it too, but you try coke after charcoal and you'll never go back, the fire takes more managing with coke but it's worth the effort to learn to control a coke fire, especially if you want to do bigger chunks of metal or forge welding, the power difference is huge, plus the metal comes out of a coke fire much cleaner than a charcoal one.