Enter the postcode of the flat here:
http://www.samknows.c...dband/exchange_search
When you see the name of the nearest exchange, click on it.
Look down the column headed 'LLU operator presence'. That indicates which (if any) third party companies use 'Local Loop Unbundling' with that exchange. Such companies can install their own equipment and cabling. If, as with my local exchange, there are no companies shown as 'Available', any ISP you sign up to will have to take BT's wholesale product (using BT equipment and engineers).
Chris