Did you tell Chase you were in London? Some years back my card was initially denied in England because their fraud detection program sees the card being used in a different location without notice and refuses to accept the transaction.
I was in London last week and had no problem. There's a place on the Chase website to enter travel plans, and after doing that I got an automated phone call the next morning saying "we've been notified of travel plans, if you did not tell us this please call, if you did tell us this you don't need to call."
At the last minute I called Chase to ask about a new card with a chip like the ones they use in Europe, although as reported elsewhere on FT the US one is not "chip and pin" like over there, but "chip and signature." They sent the new card overnight and I had absolutely no problems with it. I was told that I can still use it here as well because it has a magnetic strip on it as well, but now that I think about it I haven't tried to use it since I've been back.
It seems the new card was overkill, at one pub the server asked if we were American, saying she had originally thought we were Canadian because Americans do not generally have the chip cards. She told us most places have card "swipers" for reading the magnetic strips as well as the chip readers, which is what leads me to ask if Chase knows where you are.