Tax on UK assets being declared

Hello

I wonder if someone can help me with a tax question. I have been doing some research and found mixed signals concerning Spanish tax due on UK assets.

I intend to move to Murcia over the summer with my wife (from Murcia) and our two daughters. I intend to register as a resident as I will be staying in Murcia at least 9 months of the year and 3 months back in the UK.

I will continue to work for a UK company remotely in Spain and pay my UK taxes.

Income Tax: I have worked out (https://www.uktaxcalculators.co.uk/worl … nst/spain/) I will need to pay an additional £3500 in income tax in Spain as the income tax rate is higher than in the UK.

The bit I am confused with is declaring my worldwide income to Spain. I have assets in the UK above the 50k limit in all categories and cannot understand how Spain calculates the tax due. e.g. I have the following

ISA
SIPP
P2P
Cash in Bank

For the ISA/SIPP how do they calculate the 'gain'? I am mid 30's and cannot touch the pension and I never withdraw from the ISA's. Do they just calculate the increase in value every year and then apply their income tax? As there is no tax in the UK for SIPPS/ISA at this point in my life I would have to pay the full Spanish income tax on the gains

For the P2P/cash interest, I assume I can offset the UK tax with what's owed in Spain and pay the difference as I am doing with my PAYE income?

Thanks