Perhaps someone else covered this, so I apologize if I repeat.

There will be a Great Britain for many millions of years, because Great Britain is the name of an island. It is the mass of land with England & Cornwall on the bottom, Scotland on the top and Wales sticking out on the west edge.

If every human there were wiped out, it would still be Great Britain. If the glaciers bury it, it will still be Great Britain. It's not a society, it's a place. It's limestone, marl, volcanics, gneiss, etc, emplaced on a base of granite.

One way or another, it will be around until the sun envelops and vaporizes it, far, far in the future.