Re: Difficult to compare BT and VM
VM's network is ONLY used by VM.
BT's network (from the exchange to the punter) is by law open to being used by all sorts of other carrier. eg Sky, TT etc etc.
I think your logic is faulty. Take the local high st. in a neighbouring town. The business rates on a shop unit adjacent to the market sq. that was a hairdressers with 4 chairs is circa £80,000 pa. So are you suggesting that if I use that unit (as the previous business did) and operate it as a hairdresser's with four chairs then I should be charged the full rate (ie. what you are implying for VM); whereas if I were to allow third-party hairdressers to use the chairs, I should be charged a reduced rate as it would be difficult to apportion what should be offloaded (ie. what you are implying for BT).
Sorry, in my book the duct is the premises and hence subject to VOA considerations. How many cables I put down it and how much I charge others to run their own cables or use my cables is a business decision of no concern to the VOA. As for BT's open access obligation, yes that should qualify them for a partial rebate, but the valuation method remains the same.
Taking the high st. metaphor a step further, there are reasons why business rates on high st. premises is higher than on out-of-town or business parks; I suggest that given the ease of access to BT's infrastructure etc., it is probably more akin to the high st. than out-of-town and hence should incur a higher rate of business rates...