Spain and the United Kingdom are differents when we speak about taxes.
The first differece, is the date to each fiscal year. While in Spain is the natural year (The first of January until the thirty first of December), in the UK it is since the sixth of April till the fifth of April (the next year).
If you are planning on living in Spain or you are yet a resident here, you must tell it to your HM Revenue and Customs office as soon as possible. Thereby, you avoid to paid taxes in the UK and paid only in Spain.
In Spain, like a resident, you will have to pay taxes for all the incomes you had (state pension, civil service pension, widow pension, salary…)
The Agencia Estatal de Administración Tributaria (AEAT) is in Spain the similar to the HM in UK. Every year in the second trimestre all fiscal residents have to submit the statement of incomes.
To make this tax liability, you need all your information about your incomes: the totallity in the previous year, from January to December.