Argentinian 707s. There were several Harrier intercepts of 707s shadowing the fleet, and opportunities to shoot - 'but the option of firing on a 707 over international waters was unacceptable to the British Government'. (Falklands. The Air War.)
On the 24th April 82 Swiss diplomats advised Argentina that the next UK intercept would produce an attack; following which the 707s backed off.