Presumably, the smaller ISPs slow down due to the huge costs of bandwidth they have to pay to BT?
Which is exactly why the better LLU providers don't suffer the same problems (as they source their bandwidth from other wholesale transit providers).
The likes of O2, BE, Sky and Talk-Talk don't have to pay BT's over-priced back-bone bandwidth, so they can either offer a cheaper or better service (their choice).
Clearly BT's bandwidth is too expensive.
If it was cheap the LLU providers would buy it (they're not stupid, after all). The fact they don't points towards BT bandwidth being too expensive.
Despite the warnings (from certain pro-BT supporters - they know who they are) many months/years back that these small-time bit players would soon fall by the wayside (because they only have a dozen customers per exchange - was one outlandish claim a while back - clearly not true, as I personally know more than 12 people with a BE/O2 connection in this town, and there's no possible way I can know all BE/O2 customers in a town with 50,000 people), the LLU providers are still going and still able to provide almost flat-out speeds 24/7 (excepting the odd network & routing issue - something BT shouldn't suffer as their networks must be so huge, they should be able to re-route traffic almost un-noticed by their users).
Ade
ADSL2+ with BE
DL Sync around 4.8Mbps
UL Sync 1088kbps
DG834GT with DGTeam firmware