Looking for a job: Remote

If you are from the UK, then you will need a working visa, which needs to be applied for in conjunction with your employer. For many roles, the company may need to prove why they couldn't employ a 'native' Spaniard though. If you want to be self-employed, then you would need an 'autonomo' visa, which would require a full business plan, evidence of offers of work, and proof that you have enough money to support you if you don't have any work.

Please note that you will probably be required to speak Spanish fluently, check that any qualifications that you need / have are accepted in Spain - and also bear in mind that salaries in Spain are only around 2/3 rds of those in the UK.