Just my half pennorth for what it's worth.......
I have been with BT for 11 years, but only because I can't get Virgin cable to be honest. I have had a reasonable service with BT blighted on just 2 occaisions.
The first was in 2010 when my broadband speed dropped to between 100-68kps, tes, down from 5Gb. BT said it was fine as "we only promise speeds up to XGb and we don't stipulate a minimum speed, so you're fine".
Obviously I wasn't fine and it took me 5 months, 8 new hubs, (when there wasn't anything wrong with any of them!), 1 visit from a technician, 14 visits from engineers, all of who said "you've got up to 7.5Gb here mate, nothing wrong with your line". Funny, then why am I only getting 90kps??? The I started posting on Twitter which coincided with yet another engineer calling. This brought a twofold benefit; one, the engineer actually listened to me, (I'm not a total dumbo with computers and braodband), and instead of simply checking my connection with his BT laptop, (and his very own BT IP address), he agreed to do his checks using my laptop. Sure enough, when using my IP address, I had crawlband!!!
So I ended up toe-to-toe with a BT lady in Belfast who had responded to my Twitter posts and said there was nothing wrong with my line. After several arguments over the phone, after each call she checked with her manager, they finally agreed that I was right and BT had capped my broadband! The result? 7.5Gb speed and reliable speed at that!!!
The other issue was last month when my wireless connection started dropping out, after a couple of calls to BTs incredibly unhelpful offshore help centre where they agreed there was possibly a fault with the BT wireless hub but refused to send me a new one, I threw in the towel and re-fitted my old faithful Netgear hub and router.
You will never get a 100% reliable provider, best thing to do is to check the OFCOM results. Bear in mind that most broadband provider use BT lines.
This is a good place to start if you are wondering which provider to go with.
http://www.thinkbroadband.com/isps.html