If you are doing what the industry calls a Working line take-over which is transferring over to SKY without a disconnect, the line therefore has to stay live meaning BT will bill you for the time it takes SKY to complete the transfer.
It's most likely a partial charge for leaving the mid-billing cycle, I would advise contacting BT to discuss this and asking them to explain the Bill to you but most likely due to the transfer of services as mentioned above.
PS, your new provider may offer to cover the cost of any exit fees as part of a new customer promo but you would need to ask them.
