You may get better luck asking this on The Aerodrome forum:
http://www.theaerodrome.com/forum/Or there is this book : The Flyers: the untold story of British and Commonwealth airmen in the Spanish Civil War and other air wars from 1919 to 1940. Swindon: B. Bridgman, 1989. I haven't read it, so I can't tell you if any British pilots flew for the nationalists