Thank you, lovely people!
You'll be delighted to know that I spent the day productively and in the service of my fellow man, assessing the various biscuits that people to whom I'm related sent me in an effort to forestall my return to Albion's gleaming shores.
I can report that Balhsen's Choco Leibniz Dark are sub-par dunkers. While the chocolate topping is superior to McVitie's when exposed to hot tea, the biscuit portion absorbs liquid at quite a terrifying rate, and would suffer from terminal structural failure within 0.68 seconds were it not to be held on by the chocolate layer. I ascribe this to the fact that they are butter biscuits.
So I think we are approaching the ideal dunkable biscuit which should be of a non-butter variety with a layer (rather than a coating) of chocolate thereupon. I am reliably informed that the new album is On Hold while

perfects the recipe for The Heartland Biscuit in the top secret GBBO (Goth? Bollocks! Bugger Off!) Kitchen.