The fastest ADSL2+ ISP will be any supplying BTw WBC, cuz that will, on good lines, reduce your NM to 3 dB with resultant faster sync speed.
What are you current router stats?
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 19 Meg WBC