I agree with what Carl said - I was going to say that I seem to vaguely remember that some non-BT ISPs who have their own hardware in the exchange and so can do whatever they like do use DHCP from a remote DHCP server to replace certain functions that are otherwise delivered by PPP. This sometimes involves extensions to DHCP, which can have all sorts of options anyway, is very extensible. That’s only for IPv4 tho. I don’t know what the story is for IPv6 with such ISPs, can’t remember. In a case like that, someone’s router, which is acting as a local DHCP(v4) server for client machines on the LAN, is in fact acting as a relay in part. I have read the odd vague mention of such set-ups, but I don’t really know much about this.
As for IPv6, I use BT and my IPv6-related stuff is iirc entirely set up by static configuration so no need for any network info services. I do get IPv6CP over PPP iirc from my ISP Andrews and Arnold, and there possibly is config info actually available through it in my case, I forget. I am also unsure as I can’t think of a single case where I use any such network-supplied info for IPv6 config. I, like most people who have PPP, do get config info supplied for IPv4 via IPCP over PPP, but I don’t think I myself even use any of it.