question about sky and BT.and Utility Warehouse telephone lines?

I have a bt phone line but pay my line rental to Utility warehouse. I want to change to sky tv.broadband package but because i dont pay my line rental to BT they cant sign me up.I dont understand why I just cant keep paying line rental to UW and have my other services with SKY..any other UW members had this problem?

its a business decision not a technical one