Individual Savings Accounts
For Non Habitual Residents (NHRs), interest and dividends are tax exempt during the 10-year period but realised gains are taxed at 28%. For non-NHRs, interest, dividends and gains are taxed at 28%.
If your move to Portugal is short-term, or if you are not certain that it will be your long-term home, then there is a case for retaining your ISAs. Although you cannot add to them whilst non-UK resident, you can continue to hold them, and once you return to the UK they resume their tax-efficiency.
A planning point you may wish to consider if you have a stocks and shares ISA is to ‘rebase’ by selling and then immediately repurchasing the same funds within your ISA prior to leaving the UK to ‘wash out’ any taxable gains accrued to the point of your departure. This way, if you did decide to restructure, encash, or withdraw from the ISA as a Portuguese tax resident in the future, there would be little or no tax to pay in Portugal.
As a general guideline, if you believe your move to Portugal is long-term (as a rule of thumb, 5 years or more) then restructuring and starting an investment vehicle that is suitable for residency in Portugal would make sense for greater tax efficiency, amongst other reasons. If this is the case, planning well in advance is advantageous, as there is no tax on ISA closure for UK residents.