Here is my story.
Signed up for max speed and was guarentted it by the rep (i now know that was a lie) and got it for 2 years then there was a problem at the exchange i had sub 5mb for a couple of days and then they "fixed" it and i got just under 40 i contacted bt who completly agreed and sent round a engineer who fiddled/tweaked and reset it and i was up to 58 mb and there i have stayed for another 1 1/2 years as bt wont fix it cause they class it as above the 57mb minimum speed (even though i go under it from time to time) and i have tried everything at my end home hub 5's the Asus DSL-AC68U whale (which is currently broken with asus still trying to fix it with new firmware upgrades).
I have also upgraded my cables to cat 6a and also gotten a killer network card (they have helped ping but that's it) and i also had an engineer round at one stage as my line developed a problem with disconnecting new plate/reset fixed it (that was 1 year ago) and i asked him about my reduction in speed he blamed distance and the fact that infinity usage had increased in my area i told him that i had gotten perfect speed 76/19 he didnt seem to believe me ...
Anyway long story short i have faster internet then i had 4 years ago but i have slower internet then i had 3 years ago and bt refuse to fix it so if i was you i would get the exact speed they can give you in your contract and if they say 56mb they are giving you a generic speed and you should ask for a correct speed or there manger.
Source
It is a very generic number for somthing that is supposed to be fairly specific (agreeing to losing 3/8 of your internet at any time is just stupid)