The law of karma is a law of evolution through experience and learning rather than of reward and punishment.
This happens ordinarily through rebirth because the body and the mind being finite cannot endure the journey continuously.
Nations are different. They change and grow to express their unique soul. This change may come about through different kinds of experiences.
For example what wrong or evil karma had India incurred that it had to go through the pains of slavery? The reason was an excessive other worldliness and illusionism. Mayavada is not evil so to say but it is paralysing and takes away your contribution to the world-yagna. Hence India suffered not because of sin or evil but because it has to learn its own lesson and grow so as to occupy its rightful place in the world.
The same is happening to Britain which misused its gifts by deviating from its soul. Hence it is being forced to discover it.
Affectionately,
Alok Da