Keep in mind the islands were "recovered" (invaded) by Galtieri´s military dictatorship.
Yet I agree that once Thatcher´s government decided to dispatch its fleet to the South Atlantic, it could be argued like you say.
Remember the argentine flag hadn´t been there for a near two centuries... heck, the islands were an argentine posession from 1816 to 1832. So if there was an initial act of "imperialist agression" it was the 1833 occupation by the UK....
But by the late 20th century (1982) the island´s status quo was one of being a UK illegal occupation for over a century...
Of course I´m referring to the version of events as described in http://es.wikipedia.org/wiki/Historia_de_las_Islas_Malvinas
H*ck I was 8 yrs old in ´82.... FC