I don't think it exists. The most efficient way to grind urchin favours appears to be the favourable circumstances loop if you have high enough BDR. That's 9 actions (7 slowcakes , 1 insist on favourable and 1 for the urchin card)

if you have the Tosher affiliation it's a little more efficient as there's no need to go to the flit every 3 times as you get 7 scrap directly from the urchin card with that option.