As I mentioned on your ticket with us that will only work if the provider is routing the static IPs, or the full subnet, to you dynamic PPPoE address. Zen may do that but that last time I dealt with a BT Business connection they did not.
It's easy to test though. Just send some traffic to one of your static IPs from some external source and run a packet capture on the PPPoE and/or it's parent to see if it arrives.
You can then use VIPs as described there.
For inbound/outbound NAT you can just use the IPs directly in rules. For services on the firewall, like OpenVPN, you need a VIP.
https://forum.netgate.com/topic/80509/bt-infinity-with-block-of-5-static-ip-addresses-fibre-modem-pppoe-wan-dynamic
Steve