If you are in the UK, and only want insurance for the UK and Spain, you can re-register the vehicle in the UK as an imported vehicle. It would then be issued with a "Q" prefix number plate.
( This is relatively common across the EU. Germany (certainly used to) require vehicles owned by ex-pat residents to be re-registered with German plates, for example. See also: http://europa.eu/youreurope/citizens/vehicles/registration/taxes/index_en.htm
The vehicle is then a UK vehicle, and you can do all your insuring in the UK, or in the UK and Spain.
This is not a legal requirement as long as:
You don’t need to report the arrival of the vehicle using NOVA if any of the following apply:
the vehicle is in the UK for less than 6 months in a 12-month period and you normally live in the EU (one single visit, or several shorter visits adding up to a 6-month period)
It takes 4 weeks to complete the process, generally speaking.