In the meantime, Zen Internet confirm they will provide fibre connection to the cabinet but limited to 300Mb due they say to cabinet capacity issues, with an upgrade to 900Mb when the capacity is sorted (no date for that, unfortunately).
I think you're getting some terms mixed up. Fibre to the Cabinet (FTTP) and Fibre to the Premise (FTTC) are two different things.
FTTC only gives speeds up to 80M, as it's copper from the cabinet to your home.
FTTP is fibre all the way your home, but on the Openreach network it doesn't use cabinets at all (except in very rare circumstances). Most people can can get speeds up to 900M or 1.6G from some providers.
If Zen can only sell you 300M then it means there's a capacity issue somewhere, or (very unlikely) you were one of the very early recipients of FTTP on an ECI OLT - there are only about 50,000 of those, which means there's only a 1 in 300 chance that applies to you, and you'd probably know if you'd had FTTP available for a long time anyway.
Why don't you just order the 300/50 service? It's plenty for most households, and if you still feel the need to upgrade later you almost certainly will be able to.