For Belgium:
You’ll need to register online (free) and make sure your civil liability insurance covers your activities.
https://mobilit.belgium.be/nl/luchtvaart/drones/nieuwe_regelgeving/categorie_open/exploitant
For the UK
You’ll need to get an ‘operator ID’ online.
https://register-drones.caa.co.uk/drone-code/getting-what-you-need-to-fly
For the Netherlands:
You’ll need to register at the RDW as a drone operator.
For Luxembourg:
You’ll need to register as a drone operator.
https://dac.gouvernement.lu/fr/drones/where-when-register.html