What is the line checker estimating for the ADSL/ADSL2+ based services?
BT Retail sometimes does not like to install Infinity if the estimate is below 15 Meg, they offer a product that is physically the same, but called something else. Helps keep Infinity speeds for advertising high.
Generally the Openreach checker errs on the conservative side, but I would not bet my life on it. As of the 100's we see regrading perhaps 1 or 2 get just the estimate or lower. Best thing you can do is pace out distance to the cabinet and figure out the line length for yourself