[Cronos]

The sun did not begin to truly set on the British Empire until after the turn of the century. Probably, I would be safe in claiming the decline came part and parcel with WWI.

it didn't END until that time, but it had plateaued around the 1860s

[Racehorse]

it didn't END until that time, but it had plateaued around the 1860s

Strictly speaking, the empire did not end until Hong Kong was turned over to China in 1997.  Britain was still adding new territory to the empire through the 1890s.