Hi,
As you are out of the UK for more than 183 days, you would not be UK tax resident for the year and will therefore need to complete the residence section of the tax return.
You will not claim foreign tax credit relief for the Belgian tax but reduce the UK income that is taxable by completing the addtional information section of the return.
You can find guidance, in particular the reference to box 12, here:
Additional information notes
Thank you.