You're scared you will find it boring - do you actually want to be take maths and physics A-level, then?
If you're interested in maths, you're capable and you enjoy doing it, then pick maths and further maths. If you're sure you want to go into engineering, then especially choose further maths. As it is though, you're not sure. It also depends what field of engineering. Biology won't be much help for aerospace engineering, but if biomedical engineering is what you're interested in then it would be more helpful (as would chemistry)
They almost certainly wouldn't care about Spanish and you really should only take 4 A-levels if the 4th one is further maths. Don't take 4 distinct separate subjects, it's unnecessary and a waste of time and effort.