From my experience with over three years gametime playing France on Hard, I think GB is pretty impotent. In my game, AI controlled GB's actions against me have been three understrength invasions of Normandy and semi-blockades of my Atlantic ports. That's it after three years of gametime. The Norman invasions have consisted of single understrength stacks with too many militia units. Plus they occur too infrenquently to support one another: I crush the invasion and then months of gametime go by until another one occurs.
How the CAI has handled the Royal Navy has been my biggest disappointment. It's been a complete non-factor especially since it doesn't seem to understand that there are other bodies of water on the map other than the Bay of Biscay and the English Channel! I wouldn't mind the British army being such a non-factor if the Royal Navy was doing its thing by making me fight for my foriegn trade! It simply isn't. I collect a nice sum every turn thanks to my undisturbed Mediterranean trade nodes and routes.
I simply don't understand why NTW's CAI is so badly handling its navies. In contrast 1.5 ETW's CAI handles the naval aspects of that game pretty well: it responds to trade routes raids, does strong blockades of important enemy ports, and hunts down rival fleets with ruthless efficiency. Playing France in ETW post 1.4 patch could be a real challenge due to the Royal Navy. NTW is supposed to have a superior CAI to ETW, but, frankly, I'm not seeing that at all with regards to the naval aspect of the game. I like a lot about NTW, but impotent Great Britain with its feckless navy ranks as a disppointment.