Professor Jonathan Van-Tam said the UK's Joint Committee on Vaccination and Immunisation (JCVI) had taken the view that it was "not immunologically plausible" the vaccine would work in younger age groups and not older ones.
AstraZeneca's vaccine is among the cheapest available and considered to be easier to transport and store than rivals, such as Pfizer's vaccine, which requires ultra-cold storage.
But he said he expected many of those would decide to get the jab after "a pause for thought", adding that Covid was still a "very unsafe disease". An estimated one in 335 people in the United Kingdom had Covid-19 in the week to March 13, according to the figures.
However, he warned the United Kingdom needed to keep variants of concern "at bay" until it could update vaccines and roll them out to the whole population.
But Prof Ferguson suggested that adding France to the "red list" of countries, from which arrivals are required to quarantine in a hotel, would not be "practical" given the amount of essential trade and commerce between the United Kingdom and France.
Another potential supply difficulty could be European Commission president Ursula von der Leyen's threat to block exports of jabs from the EU to countries with higher vaccination rates that do not offer reciprocal supplies of vaccine.
France, Italy and Germany, along with Cyprus, Latvia and Lithuania, have confirmed they will resume rollout of the Oxford shot on Friday, while Spain, Portugal and the Netherlands said they will follow suit next week, although Spain said it could exclude certain groups.
The MHRA concluded that any link between the jab and clots is unproven and the benefits of the AstraZeneca vaccine outweigh any risks.
Like Mr Johnson, French prime minister Jean Castex is expected to receive the AstraZeneca vaccine on Friday.
An accumulated 659,475 people had been administered with their first shots as of midnight since late February, which accounts for 1.46 percent of the country's 52 million population, the KDCA said.
The agency said that although the vaccine was not associated with an increase in the overall risk of thromboembolic events or blood clots, awareness should be raised and information about any risks should be included in the product information.
The same advice applies if someone develops unusual bruising.
The MHRA's investigation found a "very small number of reports" of an extremely rare form of blood clot in the cerebral veins (sinus vein thrombosis, or CSVT) occurring together with lowered platelets soon after vaccination. The cases were all men aged between 19 and 59 and one was fatal.
The EMA has received an additional 13 reports of CSVT.
Why did countries stop using the AstraZeneca vaccine?
The authorities earlier said AstraZeneca vaccine doses for another 1.05 million people will arrive in the country in April under the World Health Organization's global vaccine COVAX Facility project. "While we continue to investigate these cases, as a precautionary measure we would advise anyone with a headache that lasts for more than four days after vaccination, or bruising beyond the site of vaccination after a few days, to seek medical attention", she said.