Zen market the service I'm on as "Full Fibre 900", what I can find on the speed marketing from Zen is "900mbps average download, 450mbps guaranteed download, 100mbps average upload".
This reliably speed tests via Ookla Speedtest at 910+mbps down, 110mbps up. 7ms ping times (I'm in Norwich, ping speed tests to London sites). I'm using a non-Zen supplied router (Ubiquiti USG 4 Pro). My ONT is a single port Nokia device. I'm the only user I can see in the vicinity (most of the ports around here are above ground), so I don't think there is any contention at all currently on my OLT port GPON.
So as far as the published speeds go, I'm getting them.
When I first got FTTP I saw that some of the smart queue settings in my router could not cope with full throughput (CPU bound) and so disabled them - they seem very unlikely to be needed anyway on FTTP 900 services. Also, I found that Hamachi VPN software was causing a slowdown of networking on all my Windows PCs as it made some changes to internal Windows settings (very naughty). All non-Windows devices that were suitably equipped could achieve the above speeds once the router bottlenecks were lifted. As far as consumer devices you can just buy that seem to always measure well, the AppleTV4k via its wired port seems to reliably get high results from the Ookla Speedtest app.
I note that is seems there is some equipment capability and / or equipment / ISP configuration difference that seems to give two different top speeds, and I'm not sure what that is. Some FTTP users report ~940mbps, and others are around the 910mbps mark like me (I think it's something like 913mbps). I'm not really complaining though, it makes little odds to me at that level.