About taxes, It will depend if workers of your flag carrier pay taxes in Spain or in the country of origin, like Ryanair workers do for instance.
If salaries are going to be paid under Spanish state regulations, note that taxes will be around 20/ 30 % of your gross salary approximately.
About Schools and so on, If you are going to live in either Madrid or Barcelona there are good british schools in both.
Hope that helps a bit. If not, write again, I will try to shed more light on the issue.