You don't really have a choice: a DateTime value doesn't have a format at all, it's stored as a number of ticks since a fixed point in the past. It only gets a format when you decide to convert it to a string for display. And if you don't specify a format it uses the current PC culture.
You could set the default culture for your application:
CultureInfo.DefaultThreadCurrentCulture = new CultureInfo("en-GB");
And that would force the default format to match that culture, but ... that would also affect the default culture used for parsing user input dates so it's probably a bad idea.