Oooh, research trip! I haven't done this myself (though I'd really like to do some location scouting for THE HALFWAY HERO--probably Egypt would do since I'm not going to Iraq any time soon), but I think I'd prefer to go either during planning stages, or between the first and second draft--before I get too set on anything, when there's still time that something I discover could be worked into the story easily. But it all depends on how you write, of course, and how flexible you are after your first draft.

So, er, that was a ton of help, wasn't it? ;) Hope you get to go--Britain is so lovely!