In 2013, a English mountaineer placed a flag of which micronation at the summit of Mount Everest?

Kenton Coo, a mountaineer, pitched a Sealand flag at the summit of Mount Everest. Sealand, an Oil rig in the international waters off the southeast coast of the UK was a WWII fort, and a pirate radio station. In 1968, it was declared an independent nation by its occupier Paddy Bates. Later the UK extended its international water to 12km offshore which meant the micronation was now part of the UK.

https://en.wikipedia.org/wiki/Principality_of_Sealand