oldcro
MB Enthusiast
This is correct in general, but:
Some providers have LLU (Local loop Unbundling), which means they only lease from BT the physical copper wire from the Exchange to the premises. These ISPs will have their own kit in BT's Exchange so can offer different speeds than BT do - up to the limit of the physical wire obviously.
Also, even for those who do resell the service they buy from BT Wholesale, there can be differences in speed depending on how the ISP kit upstream is busy.
So in a nutshell they all depend on BT to some extent - because BT owns the local loop - but you could still see some diversity in performance between different providers for the same premises.
A check on my local exchange services has ADSL checked and nothing else, a note also adds "This exchange has BT Wholesale as the sole provider of broadband services"
No LLU available at the exchange so from that I am assuming other ISP's have no way of increasing the broadband speed.
One of the few disadvantages of living in a remote area I'm afraid.