Hmm, very interresting post, if the person is a spanish resident which I assume they are if they are declaring all their worldwide interests, I am stunned if tax is not applicable on their savings interest(not the capital) also I believe a home kept in the Uk as a spanish resident is considered for non resident tax exactly as a Uk resident has to pay on a spanish 2nd home if they rent it then the rent is also taxable in Spain and on selling a home in Spain only over 65yr old residents are not subject to CGT but under they either have to put all moneys of selling price into another Spanish property(which therefore would not leave an amount to be returned to the UK.) This post goes against everything I,ve ever read on the subject. Of course all of this is if the assets out of spain are over 50,000€ which must be declared so very strange. I of course could be wrong but I,m sure RayD will correct me.
As to your last question I cant answer that but I,ve never read that once UK no longer in EU, residents assets out of the EU are not taxable.
Regards
Pauline.