England is a rather easy choice, here's my advice for your opening moves (not a step by step guide, just a basic outline)

Firstly you have 2 choices regarding to the expansion of your kingdom:
Option 1: Expand within the British isles and be rid of your castle in the mainland, this is less profitable but more secure, and you'll only be likely to be at war with Rebles and Scotland.
Option 2: Expand both ways, this is much more lucrative but risks war against France, Denmark and HRE, but if you can handle it then it's obviously fine.

Okay, England starts in a very promising position, within castles both across the channel, they have plenty of rebel lands to take before getting involved in a war with another major power.

Dipolmacy:
Secure an alliance with at least one of the Iberian factions, 2 if you can. Alliance with the Pope and Poland would be a massive help, the benifit from former is obvious while the latter tend to grow into an awesome Eastern powerhouse, also for some reason I found Poland to always keep their words.

Expansion:
British isles: Leave William in London, he's old and all the fighting should be done with Rufus. York is your ideal first target (you should however aviod sieging it on turn 1. On turn 2 the council of Nobles should ask you to take York with a big rewards), it's very weakly garrisoned and shouldn't put up much of a fight. The next target is less clear; you can either go for Canneveron (spell?) or Dublin, both have pros and cons. Canneveron is well garrisoned, with plenty of Welsh Longbowmen that will make siege extremly difficult, it's best to starve them out, but that takes valuable time. The alternative is you take Dublin, the city has formidable garrison in the form of Galloglaich, but they shouldn't last long against cavalry and a few mail knights should be able to take care of them easily, also the settlement has no walls. I often finds taking Dublin to be more benifical because you are able to recurit Galloglaich mercernary in Ireland, very handy troops. After that you can think about eliminating the Scots, try to do it as quick as possible to aviod angering the Pope, send a spy in Edingborough and take it in a single go. After eliminating the Scots the rest should be elementary.

Mainland Europe: Use Robert, get some mail knights from Cane and recurit some Spearmen Mercenaries, don't bother about Crossbowmen. Take Renne and get the trade going nicely. Renne has a tough garrison and should be dealt with accordingly, starve them out if you find assult too hard. Afterwards Bruges is the clear target, it has some fearsome rebels (I think they have Flemish Pikemen + Crossbowmen), the best way is to starve them out if you want to aviod high casulties. Bruges makes plenty of money and boost your trades significantly. After taking these three mainland settlements + uniting the British isles you should of estalished yourself and the rest is your choice.

Further Expansion:
Most people finds Atwerp a tempting target, my advice is don't be tempted. Atwerp is a cannon-foder that would get you a war with either Denmark or HRE or both (even if you don't take Atwerp then you still will probably at war with one of them, Denmark most of the time). After taking all of rebels and eliminating Scotland you should concentrate on France, it will be tough, but at the end. You will succed.

What to build?
In cities trade is your priority, roads and other economical structures is a must. In castles you should be building missile ranges and infantry barracks. England have top-notch missile infantry plus excellent selection of heavy infantry (pity there lacks good spears). There cavalry also leaves much to be desired.

Extra tip: Try to limit the numbers of castles in your provinces. As far as I'm concerned you only need two. Nottingham and Caen. Caen is in a good position to reinforce all of your mainland provinces while Nottingham will play and intrustmental role in your effort to unit the Bristish isles. Keeping the number of castles to a minimum should make sure your treasurys always stays at a health amound.