If you're using any service where the lines are owned by BT (irrespective of who your ISP is), the most accurate speed test available is the one provided by BT Wholesale:
http://speedtest.btwholesale.com/
I only get around 10Mbps but that's because I'm happy to stick with an 'ADSL 2+' service. (There's only me in the house so, unlike a house full of teenagers, there aren't lots of people all trying to stream video content simultaneously). I wouldn't really see much difference if I had a far faster download speed available to me.
I experience very few problems but that's possibly because I'm prepared to pay for a 'business' service in order to ensure that I get a favourable contention ratio and, on the rare occasions I might need support, phone calls answered by UK staff on the first ring.