Speed you get on FTTC depends upon your distance from the cabinet, not the distance to the exchange. If you use BT Wholesales ADSL checker it will tell you your cabinet number and a reasonably accurate speed estimate. If your speeds line up with the ADSL checker, then nothing can be done. However, if they are significantly below those estimates, then that could be a sign of something on your line. At worst, it could be that your line is aluminium based for a large portion, otherwise it could be a fault, but as the OR engineer couldn't find a fault, I surmise it could just be you are getting the best from your line.