Is it possible to get a Static IP address from BT on their home FTTP 300Mbs service?
The answers you've received are "no", but you still have some other options.
You could buy the AAISP L2TP service for £10/month. Although there are traffic speed and volume limitations, you could use the AAISP address for incoming access to your servers, while outbound uses your native dynamic IPs. You might need to do a bit of funky policy routing to get the return traffic sent the right way.
You could take a cloud VPC on a static IP for £5-£10 per month, run a tunnel from your home network to it (e.g. Wireguard or OpenVPN), and reverse port forward your incoming traffic.
If you only want static IPv6 then you could use the tunnelbroker.net service from Hurricane Electric (for free). It's a bit cheeky to use them even though you already have native IPv6. Again, I'd only use the HE addresses for inbound connections.
Or you could wait until the end of your contract and move to Zen, or one of various business-focussed ISPs like Cerberus, Aquiss etc. Cerberus include one static IPv4, and extra ones are £1/month.