Hola
to answer your questions
It's mainly a there and back company, you might get one or two nightstops a month but don't count on it. Averages out at £1200 a month although can be a bit less or more (after tax). You are taxed as a UK resident (which, if you work here, you will be) which is the same as everyone else. It would be physically impossible to keep down a full time (or indeed part time) job in Spain so any thoughts of avoiding UK tax should be now got rid of!
It's not the worse job in the business, by far not the best. It's GB airways!