One think to remember, Lee, is when you size an image down, you will lose some of the information and thus, some of the clarity. If it looks better full size than resized, there are a couple of slick photoshop trick that can help a bunch. That being said, the clarity in that JU-52 shot above is very good.
f6 on that lens is going to be a tough aperture, especially with the often cloudy skies in England. If you have a bright sunny day, it will do well, but overcast and hazy may prove a tough catch at that aperture. One thing that you also have to watch with lenses that size is heat shimmer. Even at 300, I have had heat shimmer totally obscure the focus on shots.