If you hold a valid M-card or Annex 56 (which proves you applied for the M-card), you have unlimited access to the Belgian labour market. This means you can work as an employee for any employer and you are also allowed to pursue self-employed professional activities without securing separate immigration authorisation.
However, these rights will be limited only to the Belgian territory. If you travel to other Schengen or EU countries, you will have no automatic work rights there. You will be subject to the rules applicable in each country. For example, each country sets its own rules for allowable business activities and business trips.
UK citizens who were frontier workers in Belgium prior to the end of the transition period (31 December 2020) were able to safeguard their rights in Belgium by applying for an N-card. An N-card does not give you residence rights in Belgium but ensures you can commute between Belgium and the United Kingdom and work in Belgium without a separate work authorisation.
Besides immigration considerations, recognition of professional qualifications should be taken into account, particularly for regulated professions where formal recognition of qualifications by state authorities is required. Since the end of the transition period (31 December 2020), the EU rules for the Mutual Recognition of Professional Qualifications no longer apply to UK citizens. However, recognition decisions concerning UK citizens made when the UK was a member of the EU continue to be valid.
UK nationals seeking recognition of their qualifications in Belgium after the end of the transition period will be treated as third country nationals and recognition will be based on the Belgian rules, even if they hold an M-Card. (For more information on applying for recognition of professional qualifications, see the section of the Brexit Helpdesk for post-Brexit arrivals).