The one thing you're missing is that BT is ADSL, so the further away from the exchange you are,the less speed you will get.

So how far from the exchange are you?

Also,if you have poor telephone wiring inside the house that could be the problem,try looking at the master socket for the telephone,there should be 2 little screws that can be removed from the faceplate,undo these and under the cover there should be another socket,the master socket,plug you're router in this then try the speedtest again, if it's better then it's your telephone wiring in the house,if it's the same then the problem lies with BT.