Mike: From next year we plan to move to Spain for 5 months of each year:
If you come to Spain intending to stay, or stay 3 months or more permanently, then you must register on the EU Citizens Register.
In such a case you may find that the EHIC does not cover you as it is for visitors only. You may need to check that out with DWP.
Also, I have known a couple of cases where the Tax Office has said that those registered on the EU Register are tax resident. That is an argumentative point though, depending on which part of the rules you rely on, but it might mean you will have to prove your situation.
PS To clarify: If you move to Spain as you said, which I do not think you mean, then you become resident immediately and must register on the EU Citizens Register.