Before you leave the UK - well before!
Super helpful member
I read lots of questions from people saying they are planning on moving to Spain, but have not seen anyone say they have taken one of the most important steps - and one that can be started years before you come - learnong Spanish.
It will be so much easier if you can manage to understand and speak (or even read) the local language. Bars and restaurants and some businesses have English-speaking staff, but very few official organisations.
If you go to courses at the Instituto Cervantes <https://www.cervantes.es/uk_ireland/learn_spanish_en.htm> you can (apart from anything else) find out how easily you learn languages.
It will not help you much with understanding local accents (Andalucia & Murcia folk refuse to pronounce the letter 's' for example) but you will understand a letter from the Ayuntamiento or the taxman. Doing a course for even a couple of years will get you started, and take you well beyond the usual "Dos cervezas por favor" and "cafe con leche".