Method # 4 (not covered in the FAQ link)
See if you can disable the DHCP server on the BT home Hub.
If you can then disable DHCP on the home hub and then enable DHCP in PiHole so it acts as the authoritative DHCP server for your LAN (ie your phones/laptops etc will get issued their ips from PiHole rather than the BT home hub). Just make sure to put the internal ip of the Home Hub as the Default Gateway oin the PiHole config. Pihole will issue its own IP to clients for DNS.
Has the advantage over the DNS method given as option 1 in the FAQ in that PiHole will see each unique device rather than all traffic coming from the home hub (if the home hub works the same way as mione in that clients still use the router for DNS and that then forwards it to the DNS servers you’ve told it to use.).
Instructions on this page on how to disable DHCP on the home hub