Yes, it can be confusing!
Some services are only available at certain (450/500) exchanges -due to the way Bulldog provide the backhaul - while others use the "normal" BT method.
If you want a service and Bulldog don't yet offer it in your area, pre register for it at
https://secure.bulldogdsl.com/shop/pre-register.asp (no commitment) as they use that to partly decide where to expand their own network (the other backhaul method etc) too