http://www.bbc.co.uk/news/business-23619032

The last time I switched it was a nightmare. BT screwed up all over the place. They could not find the line on three occasions, and then gave me conflicting information and messed up the contract set up. It took me three weeks to get the landline sorted and then a week for the broadband. So after they took over the line they then the left the line at an empty premise for a week.

I could not even start connecting the broadband until the landline was sorted.

When I switched from BT to Zen for my phone the process was entirely painless (which came as a seriously pleasant surprise).
Mind you, Zen still use the Openreach network so there was less to go wrong in the first place.

I was switching from one Open reach provider to BT, and they still messed up. My parents wanted to go to sky but Sky could not transfer their number across.