As for the rest, while your other two suggestions make a lot of sense to me, if you like photographing in low light you might want to consider increasing the proportion of lenses you have with IS (e.g., unless 17-24 matters a lot to you, the 24-105, with its good IS, might prove more useful than the 17-40) instead. Flash and tripods are all very well, but in London and Paris I doubt there are many interiors where you will be allowed to use either, let alone both (and even if they did, they're annoying to everyone else - it's hard to avoid crowds in either city, and in Paris, at least, people actually do still use churches for their intended use and don't seem entirely delighted by those who treat them as tourist attractions).