I'm fully aware of the requirements and costs for registering my ES plated car in UK but what is not clear to me is de-registering it from the Spanish system.
If I drive the car to UK and advise HMRC I am bringing it in and after the mods required and getting the test done etc, must I then de-register it in Spain or will the UK registration process filter back to the DGT for de-registration? If not what is the best way of dealing with Spain de-registration?

You'll need to go to this address...

Baja definitiva por traslado del vehículo a otro país

Then choose "Baja definitiva por traslado a otro país fuera de la UE".

You'll need a digital certificate or Clave PIN.

what you should do is to put it onto "green" (export) plates in Spain first. Then go to UK and register it there.

Thanks Dave.

Can you confirm if this plan would work? :

1. Drive to UK on ES plates.
2. Inform HMRC of the import.
3. Have the required mods and test performed whist on ES plates.
4. De-register online from ES plates.
5. Cancel ES insurance.
6. Register on UK plates.
7. Initiate UK insurance.

Is there any additional action required around the IVTM and town hall? I don’t know but recalling reading something where someone didn’t and they continued to be charged.

Good point, thanks. I'm not sure if doing the de-registration on the car automatically cancels the car tax but worth adding to my list to check with the ayuntamiento.

Once you become uk resident your spanish insurance becomes invalid and you will need to get UK insurance from a specialist uk insurance who insures imported vehicles You cannot register the car in uk without insurance and MOT as you have tax it at the same time as re registering it. (and you cannot use it until it has been re registered

Good to know. Thank you. You wouldn't happen to know of such an insurer would you?

Insurance wise when is one deemed to have become resident?

I have come across Crowthorne, Acorn and Adrian Flux so far. Crowthorne has the simplest, doesn't matter make or model of car and a flat fee for 28 days TP cover.

I I stated earlier, the legal way is to put it on to green, export plates first. This notifies Spain that it does not owe road tax, ITV nor anything else.

Yes, thanks, @snikpoh. I've been following up on that with a gestor.

The DGT web page says there are two options, green plates or getting it towed after the DGT confirm that the vehicle's number plates are decommissioned.