At the moment I'm on the Sky Unlimited package mixed with a deal where I get Sky TV and two channels (Music & Documentaries) as well as the Unlimited internet package. However, they were giving me speeds of around 2 mb/s, so after emailing them repeatedly about the net disconnecting and getting abysmal speeds, they "changed" the server or something I was on (a very ambiguous email apologising, saying they would improve my speeds) and now I'm getting closer to the speeds advertised.
So basically if you aren't getting the speeds that they advertise, email them and after about 3 or 4 emails, they'll actually then give you the speed you are meant to be on. Is pretty bastardly not giving you those speeds straight off to be honest, but I'm guessing most people don't realise.