I don't see why she couldn't register as self-employed here too, not sure about the arrangements between Spain & UK, have you looked on the HMRC.gov.uk website? There can be useful info on there, or call them for more specialist advice if it isn't clear enough.
Hi, My mum is registered as self employed in Spain, but for complicated reasons she cannot close her business but needs to continue paying her self-employment registration (like the class 2 stamps, but rather a lot more money). She also needs to pay lots of debt, so she is here trying to earn enough. The only jobs she seems to be getting are cleaning jobs, with individual houses, etc. The question is, can she register as self employed here too? could she later on bring her Spanish contributions here or would she loose them?