I think it’s inconceivable that that would be the case.
It would either be “arrivals over X age need a booster”, or more likely, waiting until everyone has been offered them prior to implementation.
Considering that the rumours of vaccine passports being updated for third doses weren’t planned until early next year, I’m going for the latter.
It would be political suicide for the government to introduce restrictions along the lines of "...all arrivals need a booster jab to avoid quarantine or testing.." without giving everyone affected the oppotunity to have a booster jab.
People under the age of 50 would resent deeply having to quarantine and take extra tests whilst those over 50 could travel quarantine free, and would make their feelings known at the ballot box.
Even if the government does say "...all arrivals over the age of 50 need a booster.." that then raises the prospect of people under the age of 50 being able to travel quarantine free without a booster, whilst those under 50 would need a booster for quarantine free travel.j
And then you have to consider what the entry requirements would be for non UK citizens, particularly from those countries where booster jabs are not being offered.
It would be absurd in the extreme if, for example, a US citizen aged 60 with two jabs could enter the UK quarantine free, whereas a UK citizen aged 60 with two jabs would have to quarantine after a two week holiday to the United States.
The alleged reason for this policy is, according to
Sajid Javid the "government source" who leaked the story, to "protect the UK against any new variants of concern from abroad". This is codswallop, as there are no known major variants of concern abroad that are not also present in the UK.
The real reason is, of course, to bully and scare people into getting their booster jabs.
Towards the same end, I see the government are also dangling the stick of "extra restrictions" for Christmas in front of us.
Look Boris, we're all sick and tired of all these restrictions and threats, and just want to get back to normal.
The sooner you realise this, the better.