surely the ISP still depends upon the BT overhead line to the house ... so no difference...?
Yes. Sort of. The maximum througput that the line will sustain to the exchange is determined by its length and condition, combined with the type of ADSL in use (ADSL Max / ADSL2 / ADSL2 + annex M) and the quality of your household internal telephone wiring.
The throughput you achieve to a (for example) broadband speed test page is dependent on far more than that. Including:
throughput in the IP network between the DSLAM in the telephone exchange and the connection from the IP network to your ISP
capacity on the connection to your ISP
capacity on your ISP's connections to the internet
capacity on the routeing from your ISP to the remote site
capacity on the links from the internet to the remote site
contention at any of those points could cause a reduction in the measured speeds.
Some ADSL ISPs use the BT line then their own equipment in the exchange and own IP network, removing the BT IP networks from the equation. That doesn't necessarily mean it will be any better as other networks can also fail, it just means problems are less likely to be reported as a failure at BT is newsworthy.
To give you an example of why internal wiring is important, when I moved in a couple of months ago I connected the router to the BT master socket with an extension reel of cable running across the hall, there being no power outlet in the hall for the router, nor anywhere sensibly close either. The sync speed settled to 2400kbps down / 1600kbps up. Last week I ran cat5e through the loft and down into the lounge and the study, connected extension sockets in both rooms and the two cables into the removable backplate of the master socket.
Result was my sync speed is now 3400kbps down / 1650kbps up. One extension lead was knocking 1Mbps off the speed the router would sync at.
I live about two miles from the exchange.