The Double Taxation Treaty was signed on October 21 1975 .
It has a clause that comes into play if you are resident both in the UK under the UK rules and in Spain under Spanish rules. The purpose of the clause is to determine in which country you will be regarded as “tax resident” as it cannot be both.
It works as follows:
If you are considered resident in both England and Spain according to each country’s own rules, you are deemed to be resident in the country in which you have a permanent home available to you.
If you have permanent homes in both countries, you are deemed to be resident in the country that is your “centre of vital interests”, i.e. the country in which your personal and economic matters are most significant.
If this proves to be uncertain, you are deemed to be resident in the country in which you have a habitual abode, however, if you have one in each country, you are then deemed to be a resident of the country of which you are a national. UK nationals will at this point be regarded as UK residents.