Better than that
I'm on the 20mbit/s service and about 90% of the time I consistently get 15-20mbit/s.
I've had similar experience with (what's now) their cable service since it was NTL / Telewest. In the last decade I've been on .5, 1, 2, 4, 10 and now 20 mbit/s and the speed has always been close to the advertised rate. The only downside is the low upload rate. It's not below the advertised rate, but I get the same UL from my BT 2mbit/s at work as I do from my 20mbit/s at home.
Perhaps it's a local issue. I only have experience in the Tyneside and Wearside areas.