Is a British citizen a resident in Belgium automatically and vice-versa?

Anyone staying in Belgium for more than three months is classified as a ‘resident’. After your arrival in Belgium, you have eight working days to go to the Office at your local municipal administration office/town hall (maison communale/gemeentehuis) to be registered on the Register and get your residence card. To find the details of your local town hall, you’ll need to contact the commune in which you’re living.
You’ll need to take along your passport, work permit (if applicable) and passport photos. You’ll be issued with a Certificate of Registration, and either an A residence card if you are staying for a specific amount of time, or a B residence card if you are allowed to stay in Belgium indefinitel

Belgium’s wide array of childcare and education facilities eases working parents and international families. In keeping with the myriad levels of national and local bureaucracy in Belgium, the state school system can seem a minefield to newcomers trying to make a choice for their children.

The first decision is whether to integrate your children into the local system or to take advantage of the many international schools in the country. This naturally depends on whether you are on a short-term contract or plan to stay in Belgium. The international option would allow your children to continue in the same education system once they return to their home country, while local schools help children integrate better into a new country.

The process for registration takes quite a few months but you must start it within 8 working days, the process includes a police visit to confirm that you are who you say you are and that you live where you say you live. Even as a British citizen you must have adequate means of support - a job, a pension or a very large pot of money - and you must have health cover (which in the vast majority of cases means joining a mutuelle - although at least some mutuelles won't complete the process until the registration process is complete because they need to know the National Numbers of all the family AND they need a Composition de Ménage certificate to confirm the composition of your household).

Once the registration process is complete you can obtain E eID cards (not A or B cards) for your family which are the essential lubricant of Belgian bureaucracy. You won't be able to register a car or obtain Belgian driving licences until you have your eID cards (despite the fact that, technically, your UK driving licences cease to be valid the day you move to Belgium). Until you have your eID cards you must carry your passports at all times.

Note that everything is different if you are employed by one of the EU institutions.

Just to mention that now you can drive legally in any EU country with a driving licence issued in another EU country.