Edinburgh is more like a weekend trip--five hours by train from Kings Cross--but it's a great city, and Edinburgh Castle alone is worth the trip. So is this little fudge shop on the Royal Mile. *grins*

Seconding other folks' recommendations of the Tate Gallery (not so much the Tate Modern, unless modern art and lots of British teens dressed in black trying to be cool is your thing) and the Imperial War Museum. Although you will cry.

You have JB tickets!!! I am green with envy.