Interesting idea, but probably a happy bi-product than the main motivation. Corporation tax is 30% in Spain and 21% in the UK, yet IAG is Spanish based.
As with many thing, one must look beyond the headlines. The effective tax rate, rather than the headline number is what most organisations look at (so start with the well know national headline corporate tax number, deduct what tax breaks exist, add on any regional or industry specific taxes etc), but it's not an exact science and some countries will "do deals".
According to one piece of research by the Institute of International and European Affairs in 2011, the UK had an effective corporate tax rate just above 23%, in Spain it was just under 21%, while Ireland's is about 12%.
JAS