I am their only presence in Spain. I get paid in £sterling and currently pay tax in the UK. I do not pay national insurance. I wish to pay tax and national insurance contributions in Spain but am not sure of the best way of doing this. I earn approximately £27000 per annum. I would prefer to pay my tax at source rather than have to deal with my own tax returns, but this needs to be simple and inexpensive to do or my employer will not do it. I need to know how much tax I would pay if I paid through my employer and also whether it would be any different if I paid it myself. If I pay it myself, is there a way of doing this monthly rather than annually.
An individual is resident in Spanish territory when any one of the following circumstances apply:
They have stayed longer than 183 days in Spanish territory over the calendar year. In order to determine the permanence in Spanish territory, occasional absences are included, except if the taxpayer accredits their residency in another country. In the case of countries or territories labelled as tax havens, the Tax Administration can demand proof of stay in that tax haven over a period of 183 days within the calendar year.
They situate the main base or centre of their activities or economic activities, directly or indirectly, in Spain.
They have dependent not legally separated spouse and/or underage children who are usually resident in Spain. This latter situation accepts evidence to the contrary.
So, you are an individual resident in Spain. You can't choose, you must pay taxes in Spain.
With the information you have given, annual income of 27000, it is impossible to give a sure answer.
If you need to specify other data it's necessary more information.