BT Wholesale* offer a deal where your Electric Eel ISP can set up a single datacenter anywhere in the UK, get two or more fiber links from that data to the BT core network, then you can sell FTTP, FTTC and ADSL to anyone with a BT line. BT Wholesale will set up what's basically** a VPN tunnel from each customer to your datacenter. It uses BT equipment to terminate the FTTP/FTTC/ADSL connection and the BT core network to transfer the data to you. You then have to buy a big Internet connection from someone else, and route your customer's traffic to & from the Internet. It doesn't much matter what technology your customer is using, it looks more-or-less the same to your ISP, although the ISP will have to pay BT a different set of charges.
There are other wholesale providers, e.g. TalkTalk, who can offer the same deal - in that case TalkTalk will use their own equipment in the exchanges to terminate the traffic, so TalkTalk only have to deal with BT Openreach not BT Wholesale.
Most small national ISPs will use a wholesale provider. That's because the cost of installing equipment in every exchange, and setting up fiber backhaul from each exchange, is prohibitive unless you can split it among a huge number of customers.
(* There are 3 relevant parts of BT: BT Retail, BT Wholesale, and BT Openreach. BT Retail sells phone & Internet services to consumers and businesses; in turn it pays BT Wholesale to use its national network infrastructure, which in turn pays BT Openreach to use its exchanges and "last mile" copper/fiber wires.)
(** Pedant's corner: It's not actually a VPN, there's no encryption and it's using standards that are moderately common amongst telcos but anyone else would consider wierd. But you get the idea).