Calling all armchair generals! Boudica's Last Stand.

[Renatus]

There is a world of difference between “position in” and “position approached by”.

Yes, it is the difference between translation and interpretation.  'Faucibus is an ablative of place.  It means 'in', not 'approached by'.  You might just get away with 'approached by', if you interpret it as meaning that Suetonius was in the defile and the Britons had to enter it to reach him but that can be misleading, as we have found out.

The wagons encircling the plain was a lucky co-incidence which helped delay the retreat but without the wagons being in a particular formation, SP would have failed.

If you want to adopt that reasoning, Suetonius did fail because, as Dio says, those who escaped were prepared to fight on and only accepted defeat when Boudica died.  However, the fact is that, even if a few more would have escaped but for the wagons, it was a major victory that broke the back of the rebellion.