I am making progress, and it is worth explaining some of the misunderstandings. Having read the General Conditions more and discussed more with OFCOM I can see some sense in their replies and some misunderstanding on my part.
However, it seems a slight mess.
They seem to only want to regulate consumer charges for these 084, 087, 09 & 118 calls, but they have made definitions that implicitly regulate inter CP charges in a subtle way.
They define that the retail service charge must be the same for all retailers. This was my original assumption and the big issue being how we find what he service charge is.
However, reading more, they define "service charge" as what we pay to the ongoing carriers.
Now, these two definitions can only be both correct if the ongoing carriers charge us exactly the "service charge" defined for each range as set by the range holder.
In fact, neither carrier does that - they both pay BT the standard service charge for each range and hence charge us that plus a small amount to cover their costs. This means that the "service charge" as defined by OFCOM is not the same for all retailers.
It is a cack-handed way to impose rules on carriers charging carriers - by making two definitions that are only non contradictory if the CPs charge each other in a certain way.
Now, this puts the carriers we use on the spot - they will have to charge us the "standard" service charge for these numbers, but they then need to negotiate a discount on what they pay BT to make their own profit on such numbers. BT may not be keen on that, but I see no reason why that should not be the case.
The other odd thing is that I, as a consumer, assumed the service charge went to the end telco, or even the operator of the number, and the access charge is what paid to get the call there. This is not the case. The way OFCOM define it the access charge only pays the originating telco. The cost of getting the call on to the end terminating telco is all taken out of the service charge. So if you use a premium rate number to pay a charity at say "£1/call plus telco access charge", that is not £1 going to the charity even though you may be paying hugely over the top of a "normal" call for the access charge element on top.
What a mess. Waiting on latest OFCOM response.
Update: Why is the BT carrier price list not being updated? Well, from what I just heard today, BT are taking the sensible view of "why the hell should we be the ones to compile this list of charges" and arguing with OFCOM too. That is hearsay, but interesting and plausible.
Update: Getting resistance from carriers, and no reply from OFCOM - this needs sorting.