Sky's checker tell me the line is already with Sky.. and I'd have to log in to check it.
If I start the order process on BTs site I get
Estimated download speed* range
Lowest assured download speed*
This is based on postcode, house number and landland number. This matches what plusnet advise.
As long as I hit those figures I'm fine. I just need to be sure as it will affect my ability to work from home.