Are you connected via wireless or ethernet? Although on-line speed tests show a downstream level this can be seriously reduced when pc's are connected wirelessly.
I had a similar problem using the livebox and plugged the router into the test socket. There was an improvement but in the end BT said there was a fault in the external wiring and actually allocated a new line. The problem is, mostly, solved. There are occasions when it is obvious that traffic management is taking place.