If you are only 58m cable distance from your street cabinet I would expect 80/20mb connection unless there is something terribly wrong with your line as there is no speed throttling. Can you post your hub/router connection stats to give more details about your current connection speed
If your line is genuinely performing much worse than it once did , whatever the reason for that is , it’s not a deliberate policy or action taken by OR or your ISP , what possible rationale could they have for doing so ?, there is no reason why an ISP would deliberately limit the sync speed of a line, however DLM ( dynamic line management ) can and does limit ‘speed’ for the sake of stability, if a defect is making the line unstable.
The DSL checker doesn’t indicate that your line is cable of speeds much higher that you currently get , if you used your phone number in the checker , try again but using the address option , post the results, the distance you are from the cabinet ( if your estimate is correct ) suggests something wrong , for comparison, I easily get 80/20 , and my cabinet is something like 80-100m away.
As requested a few times , why the reluctance to posting your router stats , that should give an indication of what your lines potential is , as well as what you are actually syncing at.
Getting an ISP to accept a fault or even arrange a visit even if they don’t consider there is any performance issue shouldn’t be as problematic as you apparently are experiencing, but there may well be a point at which you have to accept that if you report an issue , but non is found , then charges for the visit are possible, refusing this possibility will result in no visit being arranged, if you do hit a problem getting an OR visit , AFAIK, there is a broadband speed improvement visit that is chargeable if no issue is detected , or the issue is within your home ( like running a router from an poorly wired extension socket ) but is free if a network defect is found ( and rectified ), you could request one of these.
What’s your goal ? , to leave BT penalty free ?, it wouldn’t matter what ISP you used , the fundamentals would be the same regardless, the FTTC equipment and the line are Openreach supplied and obviously would be the same irrespective of the ISP , there could be a minor difference as far as the router the ISP supply’s but generally the speed would be pretty much the same if a different ISP were providing your service and not BT , if the line has a defect, that would still be present.