Hi CPR2020,
As you say, you are fiscal residents in Spain, that is the country where you have your 'centre of vital interest'. I assume you completed your initial Modelo 720 when you first became tax resident here and have to submit one this year to reflect the sale of your UK property last year. There is a reason why the Modelo 720 is also known as the "declaration of worldwide assets", it is where you detail all assets you hold outside of Spain.
As you're probably aware, there is a Dual Taxation Agreement between the UK and Spain, which covers various types of tax, including CGT. As such, you can't be taxed on any profit twice, i.e. in both countries, but for the purposes of CGT, you are taxed in the country of your habitual abode, i.e. Spain.
Kind regards,
Kim