The speed you receive is a consequence of physics, you undoubtedly are on a long line , ADSL/VDSL-FTTC are rate adaptive systems , the longer the line , slower the speed has to be for stability , that’s why FTTP is superior in that line length is irrelevant.
As far as why are you charged the same as someone who gets better speed , because they are on a shorter line is the only difference between them and you, and it’s never been the case that the speed obtained is a factor in setting price , you are connected to exactly the same equipment, so the fixed costs are the same , if price were based on the assets allocated, arguably because you consume more of Openreach assets ( your copper pair cable/ line is more expensive for Openreach to supply and maintain than a shorter copper pair line) you potentially would pay more if the pricing were based on what the line costs provide….much like posting a letter , the costs are same for all customers, irrespective of the individual cost of providing the service.
If you feel the speed you get means that you have effectively a connection that can’t handle the demands you place upon it , there is no compulsion to keep it , look at satellite or mobile based broadband.
