so basically syncing at 20000 would be the max you could get on sky llu?
Not necessarily! ADSL2+ is capable of up to 24 Meg, but many ISPs, incl. BT & O2, market it as "Up to 20 Meg" so as not to raise peoples' expectations too much, but they don't actually cap it to 20 Meg.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 17 Meg Untweaked 19 Meg Tweaked WBC