Are you in a minimum contracted period with Sky ? ie have you agreed a reduced rate in return for a lower price? If you are ,they will charge you to leave early which might wipe out any savings from the BT deal.If you are in a normal rolling contract with Sky ,then you apply to BT who will make all the arrangements for the switch. They MIGHT not be able to retain your phone number. Check when you apply to BT.
Use one-by-one to get lots of discounts
Shift existing card debt to 0% interest for up to 31 months
What this means for you