Belgium to England about quarantine

UPDATE 7th August 2020:

From 04:00 on Saturday 8th August, Belgium will be removed from the exemption list, so travelers arriving from Belgium after this time will need to self-isolate.

The rest of this answer was correct at the time I wrote it, but as noted above is now out of date.

NB this answer assumes that your friend currently resides in Belgium - as mentioned in a comment, travel history and departure point are more important than nationality for quarantine restrictions in England.

Most visitors arriving in England from other countries are currently required to self-isolate for 14 days on arrival.

There is a list of departure countries that are exempt from this requirement here - this is an official list from the UK government and so is the definitive source.

This list currently includes Belgium, and so your friend will probably not have to self-isolate, subject to a number of conditions:

1. They have been in Belgium or another country on the exemption list for the last 14 days.
2. They travel to England directly or only via countries that are also on the exemption list (NB The Netherlands and France are also both on the list, which would I think cover most reasonable land / sea routes).
3. They are traveling to England specifically - different rules apply if they are traveling to Scotland, Wales or Northern Ireland.
4. The lists above could be changed at any time as the situation in different countries changes - a country that is on the exemption list today could be removed at any time and thus arrivals from or via that country would have to self-isolate.

He needs to self-isolate, and fill out and print an online form no earlier than 48 hours before arrival. It says the form must be presented at the border but in fact it's only occasionally checked by an officer before the actual border control, as the information is sent electronically to another department anyway.

Should also mention that 1. enforcement is extremely scarce (there is neither the resources nor the will lower in the hierarchy) and 2. you're allowed to go out to eat/buy food if you're staying alone, so if, against all odds, he's confronted, he can use that as a "loophole" excuse

Since rules are changing often, the best would be that you check the following page: https://covid19travel.com/#United Kingdom

... and subscribe to change alerts.