Foreign Secretary Dominic Raab has defended the "balanced, targeted and proportionate" new coronavirus measures amid criticism from some scientists.
In England, people are being told to work from home if they can and rules on face coverings have been expanded.
The measures have exposed a split among scientists. Prof John Edmunds, who advises the government, said they did not go "anywhere near far enough".
But other scientists say they hope they are a shift towards a coherent policy.
It comes as Scotland recorded 486 new cases on Wednesday - the highest daily total since the pandemic began.
First Minister Nicola Sturgeon said it was "a real cause for concern", although she added that more people were being tested now than at the peak of the outbreak.
Tighter restrictions were announced in all four UK nations on Tuesday. In England these include a 22:00 closing time for pubs and restaurants from Thursday and the number of people allowed at weddings has been halved. The fines for breaking the rules are set to increase to £200 on the first offence.
Labour leader Sir Keir Starmer has said he supports the new measures, but during Wednesday's Prime Minister's Questions he accused Boris Johnson of "losing control" of the virus and of virus testing.
Mr Johnson defended the NHS Test and Trace system, saying it was allowing the government to to see "in granular detail" where the epidemic was breaking out.
Earlier, Mr Raab said there would always be the "Goldilocks criticism - too much or too little" of the government's strategy, but that the aim was to avoid the need for more drastic action.
He said if people followed all the existing measures, the country would "get through this" and "get to Christmas not in a national lockdown".
He said there was no fixed end time to the measures - which the prime minister has warned could last up to six months.
In an interview on Radio 4's Today programme, Prof Edmunds said the comprehensive lockdown in March had brought the R number - which measures how quickly the virus is spreading - down from 2.7 to 0.7.
A large range of measures was now needed to stop the epidemic growing any further, he said, casting doubt on the changes of R being below one by Christmas. "I suspect not. There's a chance, of course there's a chance," he said.
"To slow the epidemic down will mean putting the brakes on very hard. I suspect we will see very stringent measures coming in through the UK but it will be too late," he warned.
Prof Peter Openshaw, who is also a member of the government's Scientific Advisory Group for Emergencies (Sage), said a ban on households mixing indoors in England "ought to be instituted sooner rather than later".
Speaking to Radio 5 Live's Emma Barnett, he said: "I think [a ban] may well be coming very soon. I would think if we wait two or three weeks, it will be too late."
The government's chief medical adviser, Prof Chris Whitty, is reported to believe it is inevitable England will to have to follow Scotland's latest move, according to the Times.
Another scientist, Prof Carl Heneghan, from Oxford University, said the measures seemed to show the emphasis of public policy was moving towards personal responsibility, similar to the approach taken in Sweden.
He told Today. "We're starting to understand that we're trying to control the spread of infection as opposed to suppress it."
He added it was important to give these new measures time to work, maintain a clear and consistent public message and not to panic.
Debate is only going to get more heated
The debate about what is the right thing to do is just going to get more heated in the coming days and weeks.
Cases are rising - and most experts agree the steps taken by the government, certainly in England, are only going to having a minimum effect on curbing an increase in infection rates.
What no one knows is how quickly cases will go up - and what impact that will have on hospital admissions and deaths.
If the government is determined to suppress the virus - to get the R below one as the prime minister said - more draconian measures will surely follow.
But if it was definitely going for this approach, it would surely have gone further with the restrictions it announced on Tuesday. Lockdown-style measures have a greater impact when they are taken earlier.
That is what has prompted suggestions it may be moving towards adopting the "Swedish model" of accepting some spread. This is being done in the knowledge the UK is now in a better position to protect the vulnerable and treat those who get really sick - enabling ministers to balance the risk of the virus versus the risk to the economy, society and wider health and wellbeing.
But it means the big decision is yet to come - and crucial to that will be what happens to hospital cases and deaths.
The devolved nations have their own powers over coronavirus restrictions.
Scotland's Deputy First Minister John Swinney acknowledged the new rules banning people from visiting other people's homes were "difficult and disruptive" and said they would be reviewed every three weeks.
"No one wants to have this in place a moment longer. The more public compliance, the more successful we will be," he told BBC Breakfast.
And the number of UK cases rose by 4,926 on Tuesday, government figures showed, with deaths increasing by 37.
What are the new rules?
- Office workers are being told to work from home again if possible
- Penalties for not wearing a mask or gathering in groups of more than six will increase to £200 on the first offence
- From Thursday 24 September, all pubs, bars and restaurants will be restricted to table service only. Takeaways can continue
- Also from Thursday, hospitality venues must close at 22:00 - which means shutting then, not calling for last orders
- Face coverings must be worn by all taxi passengers from Wednesday
- Retail staff and customers in indoor hospitality venues will have to wear masks from Thursday, except when seated at a table to eat or drink
- From Monday 28 September, only 15 people will be able to attend a wedding or civil partnership ceremony, in groups of six. Funerals can still take place with up to 30 people
- Also from 28 September, you can only play adult indoor sports in groups of no more than six
- The planned return of spectators to sports venues will now not go ahead from 1 October
- People across Scotland are being advised not to visit other households indoors from Wednesday 23 September onwards. This will become law from Friday
- There will be exceptions for those living alone, or alone with children, who form extended households. The rules will also not apply to couples who do not live together, to tradespeople or for the provision of informal childcare - such as by grandparents
- From Friday, pubs and restaurants will have to close by 22:00
- The first minister urged people not to book overseas travel for the October school holiday
- Pubs, cafes and restaurants in Wales will have to stop serving alcohol at 22:00 from Thursday - and sales of alcohol from off-licences and supermarkets will also be stopped after that time
- Pubs will also be limited to table service only
In Northern Ireland:
- From now, people cannot meet anyone who they don't live with inside their home
- There are only limited exceptions.
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