God, your experience reminds me of my own. My problems started when I moved flats (same property just differet flats). Took them 5 weeks to transfer my phone number because some moron forgot to log my need and after waiting 3 weeks i phoned them only to find out they had no idea i was waiting for it.
Anyway, once this was settled I phoned them and asked for higher speed ( i was on 2 mb at that time). They said they couldn't do it. Ok then, i shopped around, found sky and decided to go with them. I phoned BT and asked for my MAC. When they heard I want to switch they said they can offer me 8 mb speed at the price as before !. So i said wow ok rhen, let's go for it. Since they tried to upghrade my line everything went dead. My connection dropped from 2mb to 10k (yes 10 k !!!) and it went like that for weeks and weeks in which time i was on the phone countless times with them. When I couldn't take it anymore I told them that's it ... I am switching. funny thing was they said now that I changed my speed I had to pay them for 12 months of contract and then I could have my MAC ! Took several heated conversations with them to make them realise it wasn't my bloody fault and the guy who offered me the upgrade knew very well my line would not support that speed but went ahead with it anyway/
When i finally got rid of them I promised I will never ever go back to BT. Unfortunately my phone line is still with them and the service as get is just as bad as the broadband one. So far they took money from my account twice , over £100 pounds each time and took me weeks and weeks of waiting until i got it all back. So of course I cancelled my direct debit with them. You're not getting your hands on my money again !