Of course you still become tax resident after 180 days with all that entails, Modelo 720 (if applicable), annual tax return etc. If you fail to engage with the Spanish tax system then technically you become an 'under the radar' illegal immigrant, or in the country 'irregularly' as the Spanish term it.
Also if you hold a UK driving licence that must be exchanged for Spanish.
Not aimed at you or anyone in particular but there seems to be a widespread misconception that Freedom of movement means that you can go where you like for as long as you like thumbing your nose at the local authorities and rules as you go, it does not.