Which Sky Broadband service is it?
Sky Broadband Connect (non-LLU, re-sold BT Wholesale ADSLMax) is widely thought to be crap.
However, Sky Broadband Unlimited (Sky's LLU service) is widely thought to be excellent.
If you have Sky BB Unlimited, and the speeds are poor, then it will be due to...
1) DLM running on a new connection. Give it a chance to adjust during the training period.
2) The length of your line. This is nothing to do with Sky, and you'd have the same issue with any other ADSL ISP.
3) The quality of your domestic phone wiring. This is nothing to do with Sky, and you'd have the same issue with any other ADSL ISP.
I've had Sky BB for years now, and it's rock solid. £7.50 for truly unlimited broadband, with no usage limits, no FUP... and no STM
Sure, I only get around 6Mbps, but that's because of my line length, not Sky - it'd be the same with any other ISP... but without the low price and lack of limits.