I have had a similar issue. I initially connected to Sky Fibre at 40Mbps. I then upgraded to Pro and got 47Mbps. I then downgraded to Unlimited as I didn't think it was worth it.
More recently BT say I should now be able to get 54 Mbps. I called sky to upgrade again and was told I can't get pro and the maximum speed my line is capable of is 33Mbps. I still get 40 ?!!!
Skys systems are a joke.