Since the 70-200 f/4 IS doesn't weigh much less than the 70-300L or take up much less space, you might as well go for the latter; even if you don't need the extra reach, the 70-300 is very good at isolating subjects and creates superb bokeh at 300mm. They're both excellent lenses, but I don't think I've used my 70-200 f/4 IS since buying the 70-300L.
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).
You could always lighten your walk-around load by taking the 40mm pancake with you as well; unless you're more of a wide-angle guy, you might find that you can happily spend all day with nothing but the 70-300L on your camera, with the 40mm in your pocket just in case.
(It probably goes without saying that no matter what lenses you leave behind you'll wish you had brought at least one of them, and that there will be at least one lens you take with you that you'll never use.)
