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.
I value your opinion since you have used both. I do have the 70-200 f/4L IS.
However, to me there is difference in weight and in size. True the 70-300L can be shorter at 70mm but it is thicker! This can be a problem or not depending on the bag used. To me it is.
Also, the fact that it is thicker and (more important) the rings are reversed makes the 70-300 a more difficult to handle lens (otherwise I would think seriously about it...)
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).
Flash could be used in Madame Tussaud Museum and in some internal dark places in London Zoo.
Tripod can be used for night shots around the Big Ben, parliament and the Trafalgar square. 17-24mm is a very useful focal range too.
(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.)
That is the best comment ever and I believe it applies to all of us.
P.S Irrespective of my opinion regarding 70-300L I am very interested on your further comments on that lens.