[Turtles]

ISAs have no protection under Spanish law, so you would pay tax annually on any dividends and then capital gains tax when you sell. The only consolation is that buying accumulation funds allows tax-free reinvestment of dividends, but again you pay cgt when you sell. Manually reinvesting dividends, or getting a broker to do it will not achieve this advantage.
It is important not only to change your UK non-taxable investments before you actually move to Spain, but to do it in the previous tax year. I'd recommend bed and breakfasting ISAs to establish a more favourable floor level before the move.

 

[Turtles]

I was told by an advisor from a well known firm of ants, that a SIPP is regarded not as pension but as a simple brokerage with tax being paid on dividends received and capital gains tax on realized assets. This is consistent with a recent judgement where a SIPP was determined not be a pension in Spain, at least relation to wealth tax

Could you provide some information about this please? I can find articles referring to the wealth tax implications, but not the taxation of dividends or cgt.