According the the TTB speedtest my 40/10 FTTC is giving me about 36/9. Gospel, obvs.
Someone somewhere here linked to a BT speedtest. I did a test and it showed over 70 down. Again, over 77. Then a really low one and now 148. But always with P poor uploads of 5 or 6.
Obviously I haven't followed the instructions, am using WiFi and other folk are using the connection. Which should get a lower download. And certainly one lower than 40.
So if you want to increase your download to impossible speeds...