Nowadays is not as difficult as it was 15 years ago to get Alitalia (or Alitalia Express). However the idea of the job being very prestigious and well paid in Italy is completely wrong at the present. Besides Alitalia and Meridiana (and also AirOne) you must forget about the rest ...
However 25 years is a very good age to start ... if you consider yourself old, what should I say???
Alitalia has no base for Cabin crew in LHR. Just Rome and Milan. Another question, are you Italian? or do you speak fluently Italian?
Good luck with your exams.
Ciao
Xenia