I'd personally say either just a simple ensign (e.g the FLS British Navy NPc's 'White Ensign' style one), or the Cross of St. George.
If you were going to use animals/symbols (like for example Lions, Unicorns, Tudor Rose etc) it should probably just be that symbol/animal to prevent it looking crowded which it may if you did an 'ensign-style' sail + animal/symbol.
Coat of arms of the UK might look good.