You know fibre should be there or not - no dlm so no variations in speed in theory excluding exchange congestion... and I would hope that would not be an issue with only small numbers of subscribers currently.
That's far from reality I'm afraid.
If you want full speed tests at all times of day and full single thread speeds then you're going to be paying a lot more than BT charge for a residential service.
There's PON contention (your splitter node), exchange congestion and backhaul congestion.
That's just within BT's realm before you hit the wider internet.
I've been following 2 recent online complaints with BT FTTP where they have refused to help customers on the 900Mb package who are hitting the guaranteed minimum which is around 450Mb/s.
Both cases hit a deadlock, went to the ombudsman and the ombudsman has ruled in BT's favour.
BT won't entertain your complaint until you connect the Smart Hub 2.
It has a built in speed tester that specifically tests throughput to the Hub. It rules out any and all hardware at the customer end like underpowered PC's/laptops/phones or things like problematic Ethernet adapters.
There's no DLM to affect the sync speed but it's very much a residential service and expectations should match that.
It's a contended service and is advertised/sold/priced at such.
You aren't likely to get far with any complaints about speeds under the minimum guarantee.
It also isn't a pitiful guarantee. What BT guarantee you is considerably higher than what OpenReach guarantee BT.
It's higher than I've seen from any other residential OpenReach FTTP provider.