Slitherine

I first time captured England. After that, during few turns, Britain loss morale. Why this happen and what it means?

exactly as it says - not enough British units in Britain so morale drops

The reason, as I see it, is that even though England has fallen, the British manpower pool stays constant (for whatever reason, losing cities doesn't affect your manpower reserves), so you have to have some way to simulate that the British are in a dire situation.

Jyri