Offcourse UK pilots would try to learn, and most would never realise a proficient level of spanish. At least not proficient enough to talk about non-standard stuff.
I absolutely agree with you that one common language is the safest. But face it...... it will never happen!!!!
At the time the 5 (?) ICAO languages were determined everybody knew that 1language would be safer. But aviation outside the flightdeck/control tower is more about politics then safety anyhow!
And if spanish would become the international language for r/t most UK pilots would start talking in english as soon as they would fly into UK airspace. Especially when there is a need to talk about non-standard stuff.
Thus creating the same problems we have in france and spain at the moment with the english language.