Belgium - UK - Quaratine.

If you travel to the UK by car, you have to remain in quarantine for all of the first 14 (or less) days that you are there (making a one-day trip a bit of a nonsense unless someone is going to hand you something through the window of your car - you can't go shopping or to a meeting). When you return you will have been out of Belgium for less than 48 hours so the quarantine advice doesn't apply.

You must observe the quarantine rules which means meeting no-one.

Of course the UK are not held captive in the UK for 14 days as suggested.

This is a repetition of information already on the gov.uk website.

Since last Saturday, Belgium is no longer on the exemption list so you would have to self isolate.

"When you arrive in the UK, you will not be allowed to leave the place where you’re staying for the first 14 days you’re in the UK (known as ‘self-isolating’) unless you’re arriving from an exempt country .......... If you’re travelling to the UK for less than 14 days, you will be expected to self-isolate for the length of your stay."

https://www.gov.uk/government/publications/coronavirus-covid-19-how-to-s...