So I don't have the TomTom data but I used an average speed limit and drive time of 30 mins to calculate the distance. Albeit I don't have the same maps available, I seem to have come out with a similar looking shape to everyone else!
took a bit of a shortcut by borrowing the drivemile assumptions from earlier commenters (using 16 miles instead of 30 min drivetime, since I don't have that dataset). definitely results in a larger square area than drivetime seems to.