I think you really need to ask her what this break really means. I personally wouldn't be comfortable with my other half going to London and spending 2 nights with someone who obviously means something to them.
I had a break, well actually a break up, with my fiance last year for 2 months. I too wanted to take stock, our relationship had grown stale and I didn't see any future for us. However 2 months later we got back together and are now stronger than ever, we both learned alot in that break and learned to appreciate each other.
Breaks aren't necessarily bad things, but it does sound like you need to address the situation now, before it get's out of hand. Unfortunately it sounds like she is very confused and may be thinking the grass is greener...
Good Luck