Don't they all use the same equipment from their local exchanges, the only difference in ISP's is the ratio's they run on for users per line?
Not quite true (i know all this because of the Tiscali forums), when an ISP takes over the BT exchange it moves in it's own equipment (this is incredibly expensive BTW).
IPstream is "managed" by BT and BT are actually very good at managing their exchanges, if they oversubscribe usage they upgrade the exchange.
With LLU or datastream an ISP leases X amount of capacity and tries to add every man and his dog onto it, this at peak times throttles the connection but it is the ISP's fault and not BT's.
Many ISP's just ignore or bulls**t customers instead of upgrading their equipment.