I do not understand what is the point of publishing such an ad on an Italian site, mainly read by Italian teachers, if you only consider teachers residing in the UK.
Millions of people are recruited every year from abroad all over the world. Nowadays you have flights to anywhere at any time and excluding non resident teachers just sounds like an overt discrimination.
Teacher qualifications and teaching experience acquired in Italy or in another country are of no lesser quality than the ones obtained in the UK.
Teaching issues with teenagers are the same everywhere, we also have bullying and challenging behaviours and are trained and able to tackle them. Effective methods work with anyone and what makes a good teacher is not where they were trained or educated but how effectively they cope with teaching issues and how effective their method is. Moreover a teacher coming from abroad is enriching to the students and to the institution itself. No one can argue that it is much better to have a professional teacher who speaks the language with a mother-tongue accent than one who was trained in the UK but has a strong accent, as is often the case of English mother-tongue teachers.
I have teaching experience both in Italy and England and I can say that, at least as far as Modern Foreign Languages are concerned, teachers in England are not better than the ones working in Italy: I have seen so many times English teachers teaching foreign languages in English and not in the subject language.