Gainsborough
Hi, I assume you intend taking your Spanish reg car to UK for a period longer than three months
Some years ago I discovered that by law in the EU one has third party (obligatory) cover for the life of the policy at no extra charge, in all EU and some EEA countries. So includes UK for now.
Maybe that still applies. If it does then unless you want move cover that should work.
Ask you insurers and if they are reluctant to tell you demand that they inform you by email, letter etc. If they are lying they will not put it in writing
PS Just checked here. It still applies https://www.moneysupermarket.com/car-insurance/european-cover/