"What if Napoleon invaded Britain?" My answer to that is what if Britain invaded Prussia? Or Austria? The Warband campaign sandbox format is what people love. I think you should keep the campaign simple, but expansive. Of course, someone can always make a completely scripted campaign, but a simplecampaign would be perfect for a start.
As for the question of militia, Russians would obviously have partisans, but for other countries, a simple line unit would be fine. This is for if you dong want to add any units. If you do, a militia for Britain and other countries could be farmers with, say, hunting rifles and pitchforks.