If I remember correctly Sky will only show up to 40Mb on their checker as that is the only initial speed they sell to new customers. You have to wait around a few weeks to a month before you can call up and ask for the Pro 80/20. Think it is down to the fibre modem optimising the best settings for your line and then giving a better estimate of what speed you will get.
The speed checkers are the same otherwise and if the BT says you will get 80, then you can get that with Sky too.
From your house the phone line goes to the original phone box outside, which is then connected to the taller fibre box next to it. The distance from your house to there is usually what dictates what speed you can get, so as you are so close, top speeds are pretty much guaranteed
Both BT and Sky have the unlimited packages, so you can decide by price/tv packages. I have Sky and the service and support is superb.