I've seen loads of FTTP tests on speedtest.net with 5ms pings
Your latency is related to where in the country you are The further from London, the higher the base latency.
I can't get 5ms on my FTTP without breaking the law of physics.
Ignore the latency you see others getting.
Some BT FTTP customers will get 940Mb/s 24 hours a day, 7 days a week.
Others top out around 890Mb/s and have never seen above 900Mb/s.
That's down to differences in OLT/L2S config.
900Mbps on the head which is expected with full-fibre
That isn't too be expected. You asked that very question before ordering and were told what to expect.
It's a residential service with a guarantee of 450Mb/s.
at least 850Mbps if 900 is the cap? (yet router says 1000Mbps lol)
The cap is 1000Mb/s.
Overheads mean the theoretical maximum you should see is around 942Mb/s.
As I mentioned above some Openreach/BT Wholesale config can limit this to around 890Mb/s for some customers.
The router reports the link speed between the ONT and routers gigabit Ethernet ports.
To be accurate you need to run a speed test from your BT account with the Smart Hub 2 connected. This tests the speed to the Hub alone, ruling out all of your hardware.
If that reports higher than you are seeing them you have a hardware/config issue.
BT won't entertain any complaints about speeds above 450Mb/s anyway.