Hi Craig - I asked this question at a UK consul roadshow a couple of years ago, as I am autonomo as well, but don´t intend to work the 15 years in Spain which is needed to qualify for a Spanish pension.
They said that my 30 years in the UK would be added to any years here, so the 15 year rule did not apply. Once you have made any payments into the Spanish system, then Spain will process your pension, but the UK government will pay Spain the part that they owe you, and Spain will then top up.
As Formentera Costa said, you can get a UK pension forecast which will show what they will give you, but I don´t know the mechanism for making the claim. They keep changing the age where you can claim anyway, so it is always worth asking them where you stand and at what age are you eligible to start claiming.