Hi there and welcome to this forum!
I live in the Madrid area which is pretty dry and of course the south of Spain is even drier. Almería desert was the area where many spaghetti westerns were shot
Spaghetti Western - Wikipedia, the free encyclopedia
So I think this area is not for you. How about Madrid and to the North? True it's not so easy to find Brits in these areas,but there is more water! I bet there are a few British immigrants in the Santander area because of the ferry. The weather is not so sunny in the north either, but it's probably got double the sun that you get in the UK!! And the sun can be too much in the summer, especially where there's humidty ie near the coast.
I expect there are other areas nearer the south, but I don't know the south very well.
The Tajo and the Duero are two of the biggest rivers in Spain and there are some beautiful areas near them. Try Miranda del duero, although I'm not sure if that's Portugal. It's on the border...