Legally avoiding foreign Tax on Junior and Investment ISA while living abroad

I love the ISA investment account and I’m now considering investing in a Junior ISA for my newborn son.

I understand that if you have any these accounts open while you live abroad then you are not able to make any new addition of money while you are still a non tax resident. That is reasonable and fine.

Also, if you withdraw money from the account while living abroad, there is no UK tax on it but apparently there could be in the country where you live or where you are tax resident.

In my case would be in Spain and my question is about whether the tax would be on the actual capital gain on the account or on the underlying shares or dividends within the account?

For example let’s say I have put £100K in the investment ISA during the past few years and when I live abroad, I withdraw all the money for a total value of 150K. Do I have to pay capital gains on the 50K profit? Or would I have to pay tax on the capital gains and dividends that the underlying investment generated during the years that I have been abroad?