Paris > Marseille by train is 3:08, not 4:00.
Nice writup, thanks for sharing.
The problem is we don't live near Euston station, it would take about 1.5 hours to get to Waterloo then maybe 30 minutes to get across London on the underground. With two small children and the stuff they require for a week it would be excruciating. When we get to the other end we wouldn't have a car to visit the family members were traveling to see and realistically would have to rent a car.
I've done the journey by train more times than I can count, both when I was single and before we had kids. I would be happy to do it again but the cost is easily 5x what it would be to just drive and is far less flexible.