Tax resident

I am a Spanish lawyer based in Marbella hence Spanish tax resident. I am also a UK Solicitor and since August 2010 a UK Notary Public. Only since July 2012 I have started practicing as a a sole practitioner Notary and have started invoicing (modest amounts as it is just the beginning).

I tried to apply for a NI number in September 2012 but my application was refused as they said I had not demonstrated my activity. Obviously as a sole practitioner, I have no employment contract.

I spend in Spain most of my time and my main source of income is in Spain, my wife and children also live in Spain but I fly to the UK approximately one week every month to attend my UK clients and I do not envisaging spending more time in the UK.

I am unsure whether according to the UK-Spain Double Tax Treaty, I effectively need to declare my UK income in the UK and eventually pay taxes there ,which may however be the case (section 14). 

Just in case, I have booked another appointment on 11th February to be assigned an NI number as I understand the financial year ends 31st March and I trust that I will now have more evidence to prove my activity and albeit modest, some income.

I would be thankful if you would can help me out with this case :)

VAT - Is Non Established UK business rule will be applied to me and do i have to register for VAT in UK?