A couple of years ago, in return for the relaxation of some of the regulation to which they had been subject since privatisation in 1984, BT gave a number of undertakings to Ofcom. This involved complete commercial isolation of the five main BT Business Lines (of which BT Wholesale is one) and strict control of the flow of commercial information both within and outside of the company.
BT Wholesale was probably subject to the most stringent of these regulations and the undertakings meant for them (among many other things) that they must treat all other carriers and service providers (including other BT Divisions such as BT Retail) equally. Also they were expressly forbidden to deal directly with retail customers in the way you suggest. This was because if they did so it could be interpreted that they were giving favourable (or more likely unfavourable) service to BT Retail�s competitors.
There is no doubt that the information you require concerning the unbundling timetable is available. However, as part of the undertakings BT had to demonstrate that they had robust systems in place to prevent any breaches occurring. Any of BT�s staff who provide commercial information to an individual retail customer could be subject to disciplinary action, which could include dismissal.
Your best bet is to approach the ISP you are thinking of using and ask them to get an estimate from BT when Local Loop Unbundling will take place in your exchange area.