How to garrison my provinces.

[Martok]

A strategy I often employ is the "Hub-Satellite" tactic:

I usually keep a minimal garrison in most of my provinces just like you're already doing. In addition, though, I also station more powerful armies (at least a full stack) in a few key provinces behind my front lines. Generally, I prefer these "hub" provinces to be fairly centrally located, so that they touch and/or are close to many of my other provinces. This way when one of my outer "satellite" provinces is invaded (and its overpowered garrison presumably forced to retreat to the castle), I can quickly dispatch one of my hub provinces' "rapid deployment" armies to relieve the siege in just 1-2 turns.

In your case, I would probably station strong armies in both Anjou (or Ile de France if it doesn't directly border the HRE), and Wessex (or Mercia, if you haven't conquered the Scots yet and are worried about them). That should give you sufficient ability to defend your lands without having to keep large and expensive garrisons in every single province. [shrug] I should note that this strategy *usually* works well for me; your milage may vary, however.