I would second scotland.
Someof the best wx is actually in winter. 60k viz low wind. And the scenery is spectacular.
Linked in as well with it being low season you can get accom quite cheap.
Inverness, Prestwick both have there own micro climate and if the east coast is horrible the west will be fine and the other way around. There will be the occassional pub day but not often.
Getting up from the South to the North is actually quite cheap if you book a couple of weeks in advance with both Flybe and Easyjet.
70 hours in 28 days is going to be a bit hard to do where ever you go. And thats even with perfect wx. Personally I would split it into 2 2 week blocks in different locations. Maybe one in scotland and one in spain. On the US side of things there are companys which will let you out of state with the aircraft how about from coast to coast below the snow line?.