If you're not already plugged into the master socket then try with your router plugged into that if possible (preferably with the faceplate removed and plugged into the test socket behind it), if it improves either try and find a way to leave the router plugged into the master socket or look at improving your internal wiring for the extensions.
Also, as above but also If it's an old installation it's worth asking them to disconnect the ring wire too as it's not needed these days and can cause a drop in a DSL signal.
If still no joy then a better quality ADSL filter may help (the ones ISPs supply for free are normally the cheapest available and not the best) and if still no joy after that I personally would get a better router, the ones BT supply are OK on a good line but not the best on low quality lines, in my experience netgears are one of the best for holding onto low quality connections.
If after all that it is still bad you may have no choice but to ask your ISP to cap the line speed at a reduced rate.