Moving paperwork advice

I don't know about a list but the formalities of moving to Belgium are few given that you are Polish.

When you leave the UK you should speak to HMRC. They may ask you to complete a P85 Leaving the UK form, or a Self-Assessment tax form at the end of the tax year (which in the UK would be 5/4/22). The latter is more likely if you're continuing to work for a UK-based employer, or your current employer is based outside the UK. This will help settle your tax affairs in the UK.

Other things to consider on leaving the UK - (1) as you leave your residence for the last time, take photos of the utility meter readings with that day's newspaper in the frame. (2) Check whether the Post Office's mail forwarding service is of use to you. (3) Contact the National Insurance Office to see how moving to Belgium affects any future UK State Pension you may be entitled to. (4) Let the DVLA know if you're taking your UK-registered vehicle with you. (5) If you're claiming any state benefits (child benefit, housing benefit, etc.) contact the relevant office to let them know the date you left the UK.

On arrival in Belgium, once in your new home, you should visit the local commune with your partner in order to register your residency. You should also notify the Belgian tax authorities that you have income from outside Belgium, so that they can issue the relevant forms at the appropriate time. You'll also need to sort out healthcare but how you do this will depend on your circumstances (some join an insurance fund, some get healthcare from their employer or their partner's employer, etc.).

You'll need to notify the Belgian authorities if you're bringing a UK-registered vehicle with you. If you hold a UK driving licence check with the commune whether it's necessary to convert this to the Belgian equivalent (since Brexit I'm not sure what the rules are).

It's honestly pretty straight forward due to your dual nationality. So long as you contact HMRC, the commune and the tax authorities in a timely manner - try not to leave it more than a couple of weeks after making the move - you shouldn't encounter any issues.

Magda do really check thoroughly as taxes are huge in belgium.