That doesn’t mean that you have to close the ISA.
I think you can keep it, you just have to declare any gains in any tax years that you are a resident of another country (and pay taxes on those years).
That way if you ever return to the UK you would still have your ISA.
I’m assuming worst case scenarios without knowing legislation of specific countries. I think it is good practice to be ready for ugly surprises. Local tax authorities may request legal, regulatory, bureaucratic actions that can be performed by educated professional practitioners only, unless the citizen is a well versed layman. The impact of these costs may be more or less damaging depending on the size of the cake.
What I’m trying to say is: do your homework before you leave the UK to another country, contact professional help, get acquainted with the legal framework in your country of destination and have your check book or debit card ready for action. Use that information when considering leaving.
There are countries where, by definition, the state is the only person with good will or good faith. There are states where authorities demand proof of innocence, flipping around the burden of proof… Even without ill intent from a specific state, all kinds of head aches may arise due to the simple fact that its legal code framework doesn’t predict n/or recognises the concept of tax wrap account potentially leading to a kafkian soup opera where the citizen would have to battle trough establish its individual rights… or choose the path of least resistance
Don’t just leave and hope everything will get sorted like a breeze. Ask for professional help
Belgium has no capital gains tax, only dividend tax so in my case (only holding growth stocks), I would prefer to keep it in the ISA rather than closing my account in the UK in one go. I’d rather gradually move my money over to my Belgian account when I know that I will definitely not return to the UK.