The 10meg target seems to say otherwise. However, until this is all ratified, we can't assume anything.
There are always a certain amount of get-out clauses that relate to congestion during peak hours.
For example, within BDUK:
Even though the normal superfast speed target is 25Mbps+ or 30Mbps+, that refers to the access capability during uncongested period. The specifications for behaviour in congested peak hour periods end up specifying something like 15Mbps for 90% of the peak period - which obviously allows it to fall below 15Mbps for 10% of the peak period.
You can bet that a USO of 10Mbps will come with similar clauses.
And you can bet that any SLA terms that trigger a "slow speed" payment will come with similar clauses ... but they might allow a period for the ISP to rectify the problem.
You can be absolutely sure that one speedtest result will not be enough to trigger a payment.