I want to keep watching bt sport by using the app will i lose access to the app by cancelling sky or can i continue using the app as i do now or do i need to make any changes to my account thanks
You'll need to modify your subscription if you only want to view via the app as there might be a price difference.
I’m assuming you are paying BT for BT Sport on Sky as you wouldn’t have access via the BT Sport app otherwise (if you pay Sky you have to watch via Sky Go in blurry vision).
In which case, your access to the app will continue because I don’t believe BT will know you have left Sky.
If you just want app access going forward, you may want to consider changing your subscription accordingly, although you may want to wait and see what the deal is with the upcoming rebrand.
BT Sport is becoming TNT Sport and moving into Discovery+, so the existing app will close.