It depends which product you are talking about...

IPStream - Normal ADSL (BT Yahoo! Freeserver, AOL etc...) throttling of the service is done in the data build on the ADSL Rack for 500, 1000 or 2000 product speeds. There isn't a rack for each product they are mixed in together, but each port on the rack is setup for each order and throttled to the correct speed.

DataStream - BT provided the SP a "pipe" up to 2Mbps and the SP does the throttling, Tiscali use this for most of their products at the moment, it allows the SP's to offer the 128kbps service over DSL technology. It also allows Bulldog to change the speed of the service though the day/night depending on how much you pay.

Currently 2Mbps is the fastest you can go, although I understand that if you are in a LLU (Local Loop Unbundled) exchange in London, Bulldog will give you upto 8Mbps!!! I guess you'd have to be on the doorstep of the exchange for that to work

Cheers

Cris.