UK ISA's and tax

There would be no tax on cashing out an ISA. Call your local tax office to confirm.

However you should have delcared your ISA every year in your Belgian tax return, and if you have been taking any interest or dividend income from the ISA (even if you haven't brought it back to Belgium and it's just accumulated in the ISA in the UK) over the years while you've been here and not declared that, then you may have another problem. You may have to declare and pay taxes on that income.

Again - call your local tax office to confirm.