This is not as simple as it seems, got this off the internet


Foreign registered vehicles which are circulating temporarily in the UK are usually exempt from UK registration and licensing. The exemption (provided under EC Directives) allows foreign vehicles to circulate within the UK without the need to register or pay certain duties in this and limits visits to six months in a twelve month period.
To qualify a visitor must have their normal residence outside the UK and live there for at least 185 days in each calendar year because of personal or occupational ties. The vehicle must also comply with the registration and licensing requirements of its home country and be insured for the duration of its stay in this country.
Anyone whose normal residence is in the UK or whose vehicle has been used in the UK for more than six months in any twelve has to make an application to register and license the vehicle here.
Just to clarify that point – any vehicle that has been used in the UK for more than 6 months in any 12 has to make an application to register the vehicle.

I know this is UK legislation but as it says it is an EU directive so should be the same in Spain even Catelonia.  The crunch is the bit about normal residence to qualify for the exemption from registration.  The moment that you change your residence from UK to Spain this exemption does not apply.  So basically if you live in Spain permanently you should register your car immediatly. Visitors who have not been in Spain for less than 6 months are not affected

Hope this helps