Hi
I can agree about the difficulty of steering with the keel up, frequently missed the trailer when there was wind and current to mess things up.
About stability, yes the P235 is perfectly stable with the keel up, however they are very buoyant, and can be flattened past 90 degrees in a very strong gust. I believe this happened to one on moorings with the keel up, the problem is that the keel then slid up (or down as it became), and the boat inverted! As there is no tide on Windemere, I would recommend leaving as much down as you can, and perhaps loosly tying the center rail to the pontoon.
Jim Hamilton