Home wiring issues
What the LGA or the media don't really mention quite often is that a lot of the speed / performance issues are due to the poor state on phone wiring beyond the master socket in the home. The BT Wholesale speed checker site is generally a pretty accurate estimate as long as there aren't wiring issues in the home and anyone can use this site or use their ISP provided version of this info when signing up.
People will often complain about poor speeds without doing even the most basic of diagnostics on their own home setup and blame the ISP or Openreach when the responsibility for any wiring issues after the master socket is the householders problem.
As an example, I recently went to resolve an issue for someone where they had upgraded to FTTC (via BT as ISP) and the estimate for their line was 40mbps download but they were getting about 15mbps. I found this was entirely due to their extension wiring being spliced in before the master socket. As soon as the extension wiring was correctly wired into the removable panel on the front of the socket and a central VDSL filter fitted then the speed went to the full 40mbps.