How is quarantine upon entering the UK enforced?

UK Border Force will check that travellers fill out a Public Health Passenger Locator form with their contact details, location address and travel plans. You may be refused permission to enter the UK (if you are not a British citizen), or fined £100 in England, £60 elsewhere, if you do not provide your contact details or do not self-isolate when you arrive in the UK. Public health authorities and/or the police have the power to make surprise visits to check that travellers are following the quarantine rules. A breach of self-isolation would be punishable by a £1,000 fixed penalty notice (£480 in Scotland) or prosecution and an unlimited fine for persistent offenders.

https://www.gov.uk/government/publications/coronavirus-covid-19-how-to-self-isolate-when-you-travel-to-the-uk/coronavirus-covid-19-how-to-self-isolate-when-you-travel-to-the-uk

https://www.telegraph.co.uk/news/2020/06/21/rules-self-quarantine-uk-14-days-travel-holiday/

This hould have been a comment, perhaps. But here's the universal truth

Just as any law violation (think about traffic...)

The same way police all around the world enforce law. They do all their routine activities to monitor their territory, spot people in violation, issue fines and, often, deliberately tell the press so that more people are aware that cops/bobbies are watching on them (deterrent effect).

Anyone in the UK, even a prime minister's advisor, can try to evade quarantine. Some were caught, some realistically not.

Just as pickpocketers are still around trying to to catch victims and not to be caught by the bobbies. And if you discover you have been driving 100mph unpunished for a few miles where no speed camera was installed, don't be proud of it, it's the rule.

It's up to the people to follow the rules. Those who don't "may", "likely" be caught. Not all the times.

Not worth trying the risk. Not ethical trying to.