It has the whiff a brexit about it. The current government is allergic to most forms of UK/EU cooperation. They don't want to give the EU extra leverage in the future, so its a means of diversification esp in the context of a lack of political will (strong NIMBY sentiment) to seriously invest in on-shore wind power in the UK. We already have 3-4 interconnects with EU countries if I recall: France, Ireland, Netherlands, ??? (a fourth?).
EDIT: Belgium and Norway is the fourth and fifth respectively, and apparently Denmark is planned.
There isn't anything wrong with diversification, and this might even be a good idea.
I'm just commenting upon the likely motivations are primarily domestic politics and geopolitics given its pretty obvious that you could achieve similar results by doing this with onshore wind and solar installations in southern Spain with lower technical and cost requirements.
Its definitely true that outside of the EU we are going to be treated differently than if we were EU members. EU members will always come first, and geopolitical spats with say Spain over Gibraltar or France over fishing that would otherwise get mediated via the EU could now be higher risk in future.
EDIT: Belgium and Norway is the fourth and fifth respectively, and apparently Denmark is planned.