Simply Googling road tax on UK car in Spain comes up with loads of sites that say a vehicle on foreign plates must be legal in its country of registration. The gov.uk site categorically states that UK registered vehicles being driven in Spain must comply with all UK requirements for road tax, MOT and 3rd party insurance. The EU site europa.eu also says that vehicles being driven in another member state must comply with the country of registration regulations regarding excise duty, roadworthy certificates and the minimum insurance requirements.
Several sites, including the AA, also says if you are driving abroad you must have road tax, MOT and minimum insurance from the country of registration. Loads of others make reference to "Spanish law requires....."etc etc.
A Spanish site, foropolicia.es, has a query about driving a car from Andorra. It states the foreign vehicle must meet the requirements of the Pricipality of Andorra in relation to vehicle excise duty, roadworthy inspections and insurance as laid down in Real Decreto de la Ley 18/2009 de 23 noviembre. It also says that, in the case of reasonable doubt (by the police) that the vehicle does not meet these requirements or that the vehicle has exceeded a six month stay, the vehicle can be impounded until such time as it meets these requirements.
It quotes loads of traffic law numbers, makes several references to "convenio nacionales " and "La Convencion de Viena 1968" and that these rules can be found in the "Boletin Oficial del Estado de 2010".
Maybe get a copy of the last reference.