I highly doubt it, The apple website only states the EE network is compatible (and not 3G only 4G).
As BT customers do not have access to the full 4G service from EE (no VoLTE, no 800mhz, no 4G+ etc) I would doubt it will be compatible. Just a guess though 🙂
No access to 800mhz ? How do you get that ? Is Bt lying to us as they state they use exactly the same frequency as EE. I would like to see evidence on that ! Or would a mod like to clarify this ?