I wonder if some of the speed loss you are seeing is due to G.INP and an incompatible modem.
This is highly likely. I, along with many others have seen their connection drop to this exact speed due to the street cabinet being G.INP enabled and having been given an ECI G.INP incompatible modem.
Got fed up with it in the end and bought a Huawei HG612 from eBay and have seen my speed and latency virtually restored overnight.
Really begrudged having to spend money sorting out Openreach's mess but it assumed would be a very long time before they pulled their fingers out and fixed it (if they ever would bother "because it still works doesn't it?")
Edited by kadison44 (Tue 26-May-15 15:53:18)