Singapore is currently allowing transit passengers who can meet all the entry requirements to their intended destination, and can produce negative COVID test results, to pass through Singapore. UK pax are allowed to Australia but must remain onboard the aircraft in Singapore and are confined to a seperate area of the cabin for the entire flight. Restrictions are gradually easing but it's early stages yet.
I'm waiting to see all the legal proceedings start against the government once those stranded abroad are able to return home. There was a case 20 years ago of an Australian citizen who mistakenly deported and ended up getting $4.5 million in compensation. I wouldn't be surprised to see a class action later this year once relative normality returns.
https://en.wikipedia.org/wiki/Vivian_Solon
A citizen has the right to return to their own country end of story. Other countries managed to provide emergency quarantine facilities at short notice even if they weren't of hotel standard. If necessary the army could have provided secure tented accommodation which would have been a better option for many than being on the street in a foreign country running out of money.