Hi ExpatInSpain,
The first thing to do is determine your UK tax residence status in the UK, in the tax year you left for Spain.
Please review the guidance at RDR3 (
RDR3 Statutory Residence Test) and take the statutory residence tests. The results of this will determine whether you need to declare your Spanish income in your self assessment tax return.
If you do, this is shown in the foreign section online or SA106 (
Self Assessment tax return forms if using a paper tax return.
Thank you.