It is a good estimate - although maybe on the low/safe side. Distance from the cabinet, quality of wiring from cabinet and two key points and also the speeds achieved by others close by or in similar situations to you will be used to provide the estimate.
6Mbps sounds low for an FTTC connection - can you give the EXACT text (cut and past) from the enquiry page.
M H C
taurus excreta cerebrum vincit