Does a hundred miles or so of land really
effect
weather fronts that much after they've crossed 3000 miles of water?

Yes. Westerlies, particularly those that lack punch (think high pressure)
give very little rain.

Why is the south east so dry compared to areas only a few hours
drive away?

It is generally low in altitude....It's well sheltered by the land from
Westerlies. SW\ S are rare to the reasons I mentioned. We had a short spell
of SW this Autumn\Winter and had lots of rain.