British Aircraft Carrier in Brooklyn?

What appeared to be a British aircraft carrier was docked tonight at Pier 7 in Brooklyn, where Atlantic Avenue meets the water.
I couldn’t get particularly close because of a signicant NYPD presence (including an anti-terrorism van complete with a comlink disk), but the ship was flying a Union Jack at the front, sported an impressive array of communications gear at the top, and looked for all the world — from street level, anyway — to have a flight deck. Didn’t catch any ship name or number, though…
Normally, Pier 7 only has the Casino St. Charles paddlewheeler docked there, so a warship was kind of odd. The usual news sources are silent about this. Anybody know anything else about it?