Work legally or illegally or for a company?

To work for a company you will almost certainly find it imposible if you don't speak Spanish. Few companies offer permanent contracts either as this exposes them to employment laws and social security payments etc. that they don't want to pay.

Legally - what skills do you have? You will need to register as 'Autonomo' (self employed) and pay 261.84 per month. You will need to find clients for whatever you have to offer. There are very few places to advertise in English to the UK residents (here is one of the very few).

Illegally - again you will need customers. You will not be able to set up trade accounts with providers so will not get good prices for materials.

Good luck with whatever direction you choose to persue. It's great living here.