Isn't this a question of "tippling points"?
Mornington Crescent isn't played between contestants. The contestants collectively play the game with the audience, which is in on the joke. (If you do not have an audience for the game to giggle along, the game has no value at all).
There are two functions here: the number of tube stations mentioned and the laughs the comedians can get along the way with simulated concentration, the sucking of teeth, the muttering about imaginary tactics ("he's got us on a diagonal. That's bloody clever!") and references to an imaginary set of rules ("Adjudication, Humph! Is that outside Stovold's Parameter?") as well as moves made with a type of stout certainty (Kew Gardens? Oxford Circus!").
At some point, when the laughs are almost on the verge on being played out and one can sense that the next ripple of laughter around the audience will be smaller than the one before, one comedian will use his or her sense of comic timing to wrap up the joke with the much-loved catchphrase "Mornington Crescent"! That is often prompted by another, more experienced member of the castle who says something like: "I think you've got them, there!). If the tipping point is missed, the chairman will often fabricate some drama (for example, by abandoning the game -
saying how much he hates when people don't take Mornington Crescent seriously).
When you are playing the game for laughs, but pretending you are not not, it is the laughs which are the key factor.