Insurance.
Unless the EU Directive has been changed, all EU companies must provide minimum cover (3rd Party) in all EU countries for the entire period of the policy.
They may not like to admit it though.
However you say you are moving to Spain, so it is as bob says .
Incidentally, if an EU citizen moves to Spain, they must immediately register on the EU Citizen’s Register, the 3 months, 183 day rules do not apply in that case.
PS A way to circumnavigate that is to come to Spain intending to stay for no more than 3 months. If during that time you decide to stay longer, you get a job or move here, it is then that you must register. That way you can drive your UK car as a non resident.