All the BT results and estimates indicate that your current line speed is not 80Mbps, despite this Plusnet claim.
Having said this your line speed is below estimates. It's possible that this is being caused by your internal wiring. The first thing to try is to remove the front from your BT master socket (you may need to remove two screws) to reveal the test socket. Plug your filter in here and do a speed test using a computer connected to the hub with an Ethernet cable and see if that is any better.