Most schools will not schedule interviews for times when faculty are not likely to be available and when student tour guides are off campus. So, Wednesday through Sunday of Thanksgiving weekend are pretty much off the schedule. That brings the number of potential interview days you'd miss down to two. In all likelihood, when invited to interview, you'll be offered several dates and you can work around the London trip. If not, you could, after being invited for interview, explain that you will be unavailable and ask to reschedule. I'd wait and see if lightening strikes and you happen to get an invitation for one of those couple of dates when you can't make it. I'd say, overall, it is rather unlikely to play out that way.
