Why doesn’t U.K. just shut borders & have mandatory quarantine?

[Singaporeslings]

I live in a country where the government shut external borders 9mths ago (similar to Australia but not quite as tough!).
Citizens are allowed back but people like me on a long-term work visa have to apply for permission to leave and re-enter.
And EVERYONE who enters has to do 14-day quarantine at a designated facility. You’re taken from airport to your allocated hotel (which you have to pay approx £1k for but have no choice in) and locked in. Sounds extreme but that (with other strict mandated measures) has kept our case/death numbers v low.
Why haven’t the U.K. govt done this yet???

List of continued exemptions, updated yesterday:

www.gov.uk/government/publications/coronavirus-covid-19-travellers-exempt-from-uk-border-rules/coronavirus-covid-19-travellers-exempt-from-uk-border-rules

A negative Covid test & isolating on arrival should be enough. I’ve flown 2 times since the March lockdown & airports (besides a layover in Amsterdam) & airplanes felt incredibly safe and clean. Plenty of people have family outside of the UK, or work in & out of the UK so how long do some of you propose closing the borders? To keep Covid & all mutated strains out, we would need to isolate as a country probably..... forever.

Thanks @notimagain