The Kingdom of Britannia have some tough ships. They also have the nice Guardian Generator which shares out shields, and gives those smaller ships an extra chance at survival. They're also masters of the torpedo, which is a weapon that maintains its lethality even as you take damage. They have access to Orion's, Agincourt's and Lord Hood's, which throw out crazy numbers of dice. The Vanguard submarine is pretty sub par. Overall the navy might lack the flash and toys of some factions, but they're solid and get the job done with that stiff upper lip attitude.
In the air, KoB are pretty poor. The Doncaster bomber is amazing and the Merlin is great for air protection and superiority, but our rotor ships are pretty bad. The Illustrious air carrier is great, and it's a tough choice between that and the Avenger.
On land, especially in Armoured Clash, KoB have great artillery supported by forward elements that can get easy access to Guardian Generators. I'd need to play around a bit more.
As for the submarine question, just ask your local players. I tend to buy multiple boxes because the models are awesome, but as long as you and yours don't care, doesn't matter what anyone else does.