True but you'd also have to get a train to London St. Pancras for the Eurostar which would probably entail some connection travel costs too - especially so if you are not in London.
Well yes, but this kind of comparisons look at only a certain route; in this case Paris-London, which means (or at least implies) central areas of both cities. Obviously, if we want to include any wider area there are too many variations to have a useful comparison.