Here we go again...
If the UK road tax has expired the vehicle is illegal throughout Europe and cannot be legally driven anywhere.
DVLA (Continuous Taxation Branch) will already have issued Notification of Fines for non-payment.
You can renew the road tax on-line, but the new certificate will be sent to the last registered UK address for the vehicle. You will have to pay any fines before they will issue the certificate as well as the normal renewal fee.
The Spanish equivalent to the UK MOT is an ITV inspection and has no validity on a non-Spanish reg vehicle, in other words it's not worth the paper it's written on for a UK reg car, unless you use the ITV cert as the first step in registering the vehicle onto Spanish plates.
If you attempt to drive back to Scotland with the vehicle in it's current untaxed condition, the NPRS cameras will flag up your vehicle as soon as you leave the ferryport always assuming the customs people at the port don't get to you first.
Once the UK MOT expires you are in deep doodoo, because, as stated, a Spanish ITV has no validity; the vehicle, if it is still on UK plates, must be returned to the UK for MOT Testing however you will not be able to legally drive it there unless you have renewed the road tax in the meantime.
If you have an accident or are stopped in Spain or in France with the vehicle in it's current untaxed condition you will meet with serious problems with the Guardia Civil Trafficos or the Gendarmes leading to a hefty fine and possible confiscation of the vehicle.
In answer to your question re Spanish road tax... yes there is, but its only charged on Spanish reg vehicles.
In your position, in the short term, renew the UK road tax online, and ask the people at your old address to forward the certificate when it arrives. Then start the reregistration process.
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