The British, impressively enough, were already aware of the King Tiger long before ever seeing it in combat, at least by May of 1944.
On July 18th, 1944, the Germans lost three King Tigers on the Western Front. It doesn't look like the British noticed them, however.
The following information was published in September of 1944. It's largely correct, but some errors are present. Complete information about the King Tiger would not be published until February of 1945.