Just following on from Zims informative post.

I disagree about Inverness. When converted to a city (after a while mind) it does pay off. I also had york pulling in well over 4k per turn once teched up a little. In fact the whole of the UK is a cash cow if run properly. My advice as England would be to consolidate the British Isles and that's it. Pull out of Caen if necessary. This way you will only need one army to defend your entire territory.

After taking over Britain, keep Nottingham a castle and convert Wales and Inverness to cities. Keep 4-5 units of decent troops in Nottingham and only have free upkeep troops in your cities, this will keep your spend on armies to a minimum. This coupled with a 10 stack of decent troops in a field army will ensure you can crush any invader.

When developing your cities, always build ports, roads, markets, and farms first, and after upgrading a city, always build the upgrades to those buildings first. Procure trade rights with everyone, and I mean everyone, this will keep the gold flowing.

One other thing to remember, and not everyone will agree. Always build farms and their upgrades as well. More people equals more taxes. Keep your taxes to low if you can, as this will encourage more growth. Once you have the money in the bank you can steamroll till your hearts content. Just remember to stop every now and again and build those roads, ports, markets and farms in your new provinces.

Enjoy!