I dont think british need a nerf. They just need to make other nations compete with british.
For instance, Japan is supposed to be a defensive nation.
The TC shooting invaders is a joke once you get higher up in ages.
Instead how about:
Stables/Garrison/Bunkers spawn 30% faster and spawn 1 additional troop than normal. + Disabling effects are 25% less effective against spawning buildings. Traps 1% more effective per age.
Chinese:
+3 citizens and +2 merc capacity instead of just 1. Mercs do 20% more damage and have 20% more hp and 2 Free random mercs per day.
Germans:
While under rally effect, troops take 50% less damage. Troop Damage +2.5% per age.
French:
-50% training time for troops and tactics. -15% troop cost. +1 troop Alliance gate space per age. -5% troop/tactic upgrade cost in Armory.
Greeks:
2% discount upgrade cost per age and 2% build time reduction per age. Increase the instant finish time to initial 30 min + 30min per age.
Koreans:
Resource buildings +50% health, war tactics +4 capacity(with the recent changes this makes more sense) 20% food/gold refunded and 35% oil refunded after being attacked. War tactics train 20% faster, tactics blessing 50% better.
Romans:
Army size +3% per age, mercs +2 slots, coalition troop size +15%, +3 troops for alliance gate.
I didnt put a ton of thought into the above suggestions, just an example of some things that could make other nations much more appealing.