BT have installed FTTP in some areas of Tenterden. FTTP stands for Fibre (optic cable) To The Cabinet. This means that there is a fibre optic cable linking your nearest green cabinet on the street to the local exchange. It does not necessarily mean that the fibre is yet installed in the footpath outside your house.
Furthermore BT will also have to then get the cable from the footpath to your property.
Any one of these reasons could be why your new broadband is being held up and of course you are dealing with BT!
Hope this is of use.
There is a very useful resource at http://www.samknows.com/broadband/exchange_search
that will give you more information.