Vehicles have been stopped on the motorway  AP7 with GB plates. They enquire whether the driver is on holiday and when did they arrive in Spain and are they on their way back to the uk etc etc. They keyed in the details and it came up on the cops computer their ferry crossing entry into Spain or France, but no return ferry was booked.

They said  though the law states you can drive on GB plates for 6 months, you must start the process within 3 months as it takes 3 months to normally complete.

The driver was not fined but handed a document to say they had 3 weeks  to start the process or their would be consequences. Before the 3 weeks were up the owner received written notification to warn him that he had not started this ball rolling and to do so immediately!

Another factor to be aware of:

If you have imported a vehicle from GB into another EU country eg Germany and paid Importation tax to them and then relocate to Spain on different number plates, not GB. You do not have the 3 months window! In fact you have to change your matriculation over in less than 2 months in total completion or you are stung for importation tax on the vehicle in the EU for a 2nd time and at the % the government lays down that vehicle is valued at- in addition to all other charges.
This is  fact!!! :(