Actually according to the UK Govt, that isn't the case and I quote using official guidelines
here:
Entering Belgium from the UK
The UK is currently classified as red zone (high risk) on the Belgian government’s travel colour code system. If you do not hold a Belgian, EU Digital COVID Certificate or a NHS COVID Pass with a QR code (England, Wales and Northern Ireland only), you are considered to be unvaccinated by the Belgian government and should follow entry requirements for unvaccinated travellers.
Travellers who are fully vaccinated with a Belgian, EU Vaccination Certificate or an NHS COVID Pass (England, Wales and Northern Ireland only)
You must:
- complete a Passenger Locator Form (PLF), unless you fall under a very limited list of exemptions listed on the Belgian government website (under the Transport and International section)
- take a test on day 1 or 2 after arrival and quarantine until you receive a negative result. You must take a further test on day 7. If any of the test results are positive, you must self-isolate.
Further details are listed on the
Belgian government’s website.
Travellers who are not fully vaccinated, or vaccinated with a vaccine or certificate not recognised by Belgium
If you fall under this category you will only be able to travel to Belgium for a very limited set of reasons, set out on the
Belgian government’s website (under the Transport and International section).
You must:
- carry documentation that proves your essential reason for travel. You may need an Essential Travel Certificate. Applications need to be submitted to the Embassy of Belgium in the UK at least five days before your departure
- complete a Passenger Locator Form (PLF), unless you fall under an exemption listed on the Belgian government website (under the Transport and International section)
- non-residents must provide evidence of a negative PCR test, taken within 72 hours before arrival
- quarantine for 10 days on arrival and take a PCR test on day 1 and day 7. If the day 7 result is negative, you may end quarantine early.