What do you need to be legally compliant as an operator?

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.

https://www.rdw.nl/particulier/voertuigen/drone/het-vliegbewijs-en-het-exploitantnummer-van-een-drone/aanvragen-vliegbewijs-of-exploitantnummer-van-een-drone

For Luxembourg:

You’ll need to register as a drone operator.

https://dac.gouvernement.lu/fr/drones/where-when-register.html