Tomás Cano Binder, CT wrote:
If they have a VAT number, you do not include VAT, since it would be an intracomunitary invoice to a company. If they do not have a VAT number, then the problem is for you that Spanish law requires you to include VAT but they will not accept it.
That's correct, so long as both you and the UK company are registered in the VIES EU database. If not, you have to add 21% VAT to the invoice. You can check this out here:
You only add (actually, you subtract) IRPF withholding to invoices issued to Spanish companies, not elsewhere or even to Spanish individuals, because only Spanish companies will pay this withholding to Hacienda.