What decided me to go back to BT for everything (Broadband for the first time), was the superb service I got from BT Openreach.
Openreach has no diff relationship with BT Retail, from whom you get your BB, than any other ISP. Hence this reason doesn't hold water.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC