I'm less concerned about the 452 - for the reason 99% have been so far linked.I wonder how many have resulted from the 452 unmentioned mystery cases?
That suggests 452 new chains then have begun which if it is just 2 others per household would add another 900+ cases. Or around 1,350 in total.
A subsequent case in the household is then linked to the mystery case as a household contact. If correct that suggests contact tracing is successfully identifying the source in less than 1 out of 3 new non-household/non-workplace cases.
If they wanted to address this, they could prioritise those who are positive on an asymptomatic test (and probably symptomatic but not a known close contact) because they are most likely to be 'confirmed' unlinked case.
So a) prioritise test results of known close contacts, but b) prioritise contact tracing on asymptomatic cases and symptomatic but with no known contact.
(Edit: so it says symptomatic but with no known contact)