Hello Slight,

First of all we need to be clear on the fact that you can only be a fiscal resident in one country and not in both. Let’s have a look the 2 options you are suggesting.

1.- Working as an autonomo
Any autonomo in Spain is obliged to pay social security with the exception of those with an income below the minimum salary currently at just over 600 euros per month.

2. - Drawing profit from the LLP
Please note that we are not familiar with the LLP figure. I assume it is similar to our SL companies, however it will be best to speak to you UK accountant regarding this matter.
If you received an income from an UK LLP the only form I can think of would be as dividends. Dividends are taxed in the UK, however as a Spanish fiscal resident you would still need to inform the tax office of the income (there is a double taxation agreement between the 2 countries and you will pay taxes only once). In this case you will not need to pay social security.