if nothing else it uses precious radio time. "turn right heading zero two zero" is all you need. do you turn to a flight level? to a speed? what else are you gonna turn to? the extra word 'degrees' is not necessary. if they say reduce to two one zero, does that mean descend to FL 210??? reduce heading to 210 degres? where exactly is the confusion?
Whatever you may think about some of the peculiarities of British phraseology, the decision to vary from ICAO standards is almost always based on empirical evidence of misunderstanding or ambiguity. You, as an individual, may think there is no possible way of misunderstanding whether 210 is a heading, level or speed, others with a lesser grasp of the phraseology have mistaken a clearance. Wherever possible data is collected to show that the different phraseology reduces misunderstandings or improves the situation in some way.
Well as far as I know, Spanish is an official ICAO language. Whilst an all english enviroment would be desirable, the poor atco might have to speak spanish if spoken to by the pilot ?
Spanish is one of the languages in which ICAO publishes its documents. This has nothing to do with the language used for RTF - the standard for RTF is the local language or English on request.