(In addition to all of the excellent suggestions above): May I suggest, while you prepare for your invasion of Britannia, that you blockade as many of the Briton ports as you can. Keep your fleets there and continue in blockade mode for turn after turn. This will soon produce an adverse affect upon their economy.

Also, don't overlook the benefit of using a powerful, 1-2 combo punch of sending a spy & assassin just ahead of your army(s). Assassins are a great way to conduct pre-emptive strike raids by destroying important war-producing buildings in enemy held towns prior to launching a full scale attack. I use them in this manner - and - if more conducive to the situation, I'll have them take out buildings that contribute to happiness (temples) or economy (forums, etc.). Sending a spy in first will show your assassin what buildings are present (in addition to giving you tactical awareness of the enemy composition.)

Good Luck!