It took 10 years? to get great treatment for HIV; 4100 people/year in UK still get new diagnoses each year. But HIV isn't easily contagious, it needs bodily exchange of bodily fluids not sharing the same air space.
I reckon if UK wanted to follow Taiwan scenario, on top of blowing up Ireland, they would have had to shut down about 1 Feb 2020 when prevalence was that low. Scaling up that 35,000 Aussies still stranded abroad, would mean a mere 100k Brits trapped abroad still... except Brits travel abroad a huge amount more than Aussies. So probably more like 300k people still trapped abroad, even now.
Another way to calculate... If Uk has booked 4600 rooms for the Quarantine programme, that suggests about 400 people into them each day (10 day programme). To accommodate the current 20k arrivals day, the UK would need 230,000 hotel rooms.
NZ has 4500 managed isolation rooms available, 14 days isolation, for regular population of 5million.
Scaling up but allowing only 10 days quarantine, that suggests UK would need 60,300 hotel rooms, and that means maximum of about 6000 new arrivals/day. 33% of current arrivals by air. I don't know what count is of arrivals on ferry/chunnel, but presumably some that aren't hauliers, so probably would need to reduce UK arrivals to no more than about 15% of what they actually were since Feb 2020. Upshot is I reckon UK would have had far more than 500k people still stranded abroad even in Feb 2021. How ARE Aussies managing abroad to pay their bills etc.? When they probably have no right to work where they are abroad.
& I don't know what Uk would do about all the land bridge journeys. There might be a black market smuggling ring of Irish hauliers bringing over Uk citizens trying to get back into GB. Otherwise Make people from Northern Ireland also quarantine in order to go to GB too, I guess.