Check the COVID-19 travel guidance for entering the UK.
Tell the UK and Spanish authorities if you are returning to the UK permanently.
To help prove you are now living in the UK, you should deregister with your:
local town hall (padrón)
the Spanish National Police (Residencia) (in Spanish)
your local health centre
If you get healthcare in Spain through the S1 form, you must contact the Overseas Healthcare Team on +44 (0)191 218 1999 or Seguridad Social to make sure your S1 is cancelled at the right time.
To move your pension or benefits payments to the UK, tell the International Pension Centre and the Instituto Nacional de la Seguridad Social.
Read the guidance on returning to the UK permanently which includes information on, amongst other things, bringing family members, tax and access to services.