You should get nice through put if you on one of there directly connected exchanges even if your exchange gets a lot of others users, as far as I remember they installed duel 10G connections. Most ISP's have a single 1G.
If you are on an exchange without a direct connection it's all on BT wholesale and it will be as hit and miss as it is every other ISP. (the wonders of WBMC)
Various (Dile up) -> clara.net (Dile up) -> TELE2 (Microwave) -> ZeN (ADSL)