They can't check every case at the moment. They've got a version of the PCR test that has a quirk in its operation that means it can usually spot the difference between Omicron and Delta - but that's just used to build up a statistical picture for incidence levels, by finding probables.
Actual sequencing is more limited again, and takes longer, so again that's a sampling thing, but I think all hospital cases are being sequenced.
As of Saturday, around 20% of UK cases were Omicron. In London it's thought that Omicron now accounts for maybe 80% of cases. It's assumed Omicron will be the majority of new cases across the country by the end of the week.
Not surprisingly it's London where cases in general have grown fastest in the past week or two, and most of that growth is in the 20-40 group. Other age groups, the cases are still roughly flat.