Cabinet 8 is live with FTTC and is located here
https://goo.gl/maps/HshEgMEgNsR2Putting TN30 7LP into the BT Wholesale checker (
full address not postcode only) returns a list of properties, selecting any of them gives full FTTC as being available except for two properties which return "Your address has been successfully matched; however we cannot determine ADSL availability at this address. Please try the Telephone Number or Postcode Checker."
How far are you from cab 8?
How long have you had the number at the current address and is it a BT number not LLU?
Have you tried just your postcode in the
full address checker and then selected your address from the list?
If you are on cab 8 and not out of range it could be a database error and you will need to convince your ISP to chase up BT to get it sorted.