Short answer: you need to install new mic
Long answer:
Hiya, I have a UK GT pack Zed also and have also installed an aftermarket double din headunit (HU).
Whilst installing said HU, it was said that because the existing handsfree kit could not be connected to the new HU, mainly because you wouldn't be able just to plug the existing mic into the new HU, since it is incorporated into a wiring loom.
But since my new HU came with Bluetooth built in and a new mic, it was no real hassle getting it all to work.
Tricky bit was finding a location for the new mic and then routing the mic wire back to the HU.
Choose the top of the driver side a-pillar as the location and then proceeded to route the wire behind the trim pieces. With a little gentle persuasion most trim panels will come away. All I lifted we're the a pillar trim and the trim beneath it, then routed the wire to the driver footwell and then to the centre console to the HU.
Sounds more complicated than it is.
Have fun and good luck with your install.