Johnson and other senior conservatives were widely criticised for his travel and quarantine policy earlier in the pandemic, when he kept borders open to foreign travellers even as infection rates spiralled, yielding Britain one of the world's worst per-capita death tolls from Covid.
The government controversially dropped the masks mandate in July for England, after a prior lockdown, but the devolved administrations in Scotland, Wales and Northern Ireland had kept it in place.
All four UK nations are expected to adopt the same PCR rule, after England again diverged in July by requiring only a simple lateral flow test for incoming passengers on flights, ships and trains.
Travel from 10 countries in southern Africa is now banned because of omicron, but Javid conceded that hundreds of passengers had arrived on flights from South Africa on Friday without being tested.
But he told BBC television: "I think the speed at which we acted at could not have been any faster."
'Holes in the defences'
Javid ruled out reintroducing social distancing rules and work-from-home guidance, which were also controversially discarded in England earlier this year against the advice of government scientists.
Javid said it was too early to judge the effectiveness of existing vaccines against omicron, as drugs manufacturers rush to research new treatments against the emergent strain.
But the government is seeking approval from its Joint Committee on Vaccination and Immunisation (JCVI) to expand the rollout of booster jabs, shortening the time-frame between second and third shots, and broadening the age range to all over-18s.
The JCVI is expected to respond early next week, Javid said.
He added that no further cases of Omicron had been detected in Britain, after the government on Saturday confirmed the first two cases, both linked to travel from southern Africa.
The opposition Labour party said the government was again doing too little, too late after Omicron emerged.
Even after Tuesday, passengers can enter Britain without a pre-departure test and travel freely from their port of entry on public transport, Labour's foreign affairs spokeswoman Lisa Nandy said.
"We desperately want to see them tighten up the travel restrictions," she said on Sky News.
"There is a real problem when for 18 months the government has been warned that there are holes in those defences and still hasn't taken action to plug (them)."