Probably you addressed your query to a portal addressing mainly Indian taxation. Your query can best be addressed by tax experts dealing with UK and Belgium tax laws.
In general, many countries tax their residents in respect of their global income, i.e., income earned within the country of residence as well as the income earned outside their country of residence. Further, they also levy tax based upon the source of income, i.e., all incomes arising out of transactions attributable to them, even if the persons are not residents. Thus there is an element of double taxation, in the case of residents, as they have to pay taxes based upon the residence in the country of their residence and pay taxes based upon the source, even if they are not residents of such countries. Therefore in your case, the UK authorities levied tax because of your residential status and Belgium tax authorities also levy tax because of the source of your income is because of your work in Belgium.
Normally there is a double taxation relief treaties between country to country to promote trade and commerce. Most probably UK and Belgium will be having double taxation treaty. If it is so, the taxes paid in Belgium can be claimed as credit in respect of taxes payable in UK. Thus if your tax liability in UK in respect of income earned in Belgium is say X and the tax paid in Belgium is X-Y, you can claim the credit for the X-Y amount in respect of your UK tax liability.