I've been planning to switch from BT for some time now. Not because of bandwidth throttling as such, but more because of Phorm. Until recently I discovered that BT's throttling doesn't just influence BitTorrent, but any encrypted communications on a non-standard (or unknown) port. Namely, access to my virtual server's control panel, which uses port 4643, encrypted.
Since I've had this virtual server, I've noticed that it's near impossible to access it's control panel between 5pm and around midnight. Initially I thought this was more of a problem with the provider of the virtual host, either way I wasn't too fussed, as the virtual host at present only runs SavageReactor, and my parents company site. I got a friend on O2 to try access the panel, and shockingly, it loaded fine for him. So I whacked my torrent on pause, and tried again, it loaded, it still took ages, but it did load.
Over the next few months, I'm going to be launching a new service for their company, and if there are problems with the virtual server, I need to be able to fix them ASAP, not wait around until BT stop shafting my Internet connection.
BT's fair usage policy simply isn't fair. I'm paying for an 8Mb/s Internet connection and I have no problem with them throttling it during peak times, but to go from a maximum speed of 850KB/s to 2.3KB/s is rediculous. That isn't throttling, that's as good as cutting off my Internet connection. A ten year old mobile phone supports a faster data connection than that.

To put that into perspective, during throttled times, it took me 25 minutes to download just 4MB. At those speeds, it would take you 36 hours to watch a 45 minute episode on the iPlayer (assuming 350MB per 45 minute episode). If they throttled me down to a reasonable speed, say for example 1Mb/s, that would be perfectly acceptable. It would at least be enough to use the control panel on my virtual server without issue.
But no, Granny's iPlayer is more important, so I'm forced to leave BT.
So when I do leave BT in the next few months, or sooner if they launch Phorm, the negative effects on BT will hit them and Granny, not me.
BT will loose my money, they will also loose the ability to throttle my torrents/connection, but I will still be using the same ADSL pipes that come into the village. So the extra bandwidth I do get for my torrents and my server's control panel will be taken away from Granny down the road and her iPlayer bandwidth.
All it would have taken for BT to avoid this, is to be 'fair' in their throttling, and not sign up to the likes of Phorm.
As more and more people abandon ISPs involved with Phorm and throttling, the effect on the network is only going to get worse, and ultimately the overselling of bandwidth will be revealed to Granny and the non-nerd population out there. Hopefully at which point ISPs will finally start investing in new kit, instead of boasting faster speeds, while quietly throttling people more and more.