Dear Lukas

Your girlfriend doesn't need to register in both countries. If she is already registered in Spain as sole trader, she only needs to invoice the UK Company for the work she does there.

The income generated will be included in her VAT ( IVA in Spain ) and Income Tax return Declaration ( IRPF ) here in Spain.

Anyhow, if she paid taxes in the UK, she could set these off against her Spanish tax bill. Her accountant in Spain will make all the necessary arrangements.

I recommend her to keep record of the number of days she is out of Spain working as she will get a reduction on her yearly tax.

Regards,