It doesn’t quite work that way. The Belgrano was sunk outside the 200-nautical-mile total exclusion zone around the Falklands.

Exclusion zones are historically declared for the benefit of neutral vessels; during war, under international law, the heading and location of a belligerent naval vessel has no bearing on its status.

In addition, the captain of the Belgrano, Héctor Bonzo, has testified that the attack was legitimate (as did the Argentine government in 1994).

Though the ship was outside the 200-mile exclusion zone, both sides understood that this was no longer the limit of British action — on 23 April a message was passed via the Swiss Embassy in Buenos Aires to the Argentine government, it read:

“In announcing the establishment of a Maritime Exclusion Zone around the Falkland Islands, Her Majesty’s Government made it clear that this measure was without prejudice to the right of the United Kingdom to take whatever additional measures may be needed in the exercise of its right of self-defence under Article 51 of the United Nations Charter.

From Here

I've also tried to find confirmation that Thatcher said to "Rejoice" about the sinking. I can't find it.
The only references to "Rejoice" was for the re-taking of South Georgia.