Before joining BT, I would regularly check what speed I could get with BT Infinity. The speed check on the Infinity page showed 28.8Mbps. I signed up thinking 28Mbps would be great compared to 2 - 3Mbps, but I have never been able to over 14Mbps. It is so misleading saying up to 28Mbps at point of sale and get no where near that speed.
For reasons unknown, since joining BT I am now barely able to get 13Mbps.
Why has this changed? Has this happened to anyone else?
I think the estimate is based on known line lengths etc from the cab that would ultimately serve you on Infinity. This is unfortunately only based on what records of cable lengths are to hand at the time. I think that you must be some distance from the cabinet and possibly you may have the dreaded aluminium in your service. That can be very deleterious to VDSL.
The other thing that could be causing your troubles is if you are testing wirelessly or if you have complex internal phone wiring. These two can affect the final speed that you are getting.
You could start by doing a profile test on your line and post the results here.