I had a similar problem when my BT Broadband was increased form 8Mbps to 20Mbps. It wasn't that the line was slow it kept disconnecting. I posted my query on this BT Help Forum
http://community.bt.com/ and I received help from them via that. I didn't call Mumbi Calling as I can't understand what they say and they don't understand what I say.
Your line does take time to stabalise, about 10 days, and I was advised to not switch the Home Hub off as it will never acclimatise to your line. Since leaving the Home Hub switched on 24/7 I have no problems and my line has settled down to a download speed of 10.5 Mbps and upload of 0.76Mbps. As my PC is connected to the hub via ethernet cable I disabled the wireless to save power.
Are you using BT's Speedtester or one of the many you can find on the web? BT's speedtester is found here:-
http://www.speedtester.bt.com/ give it a try and see what you get.