Medicine Market is the root of the whole gang upgrade system: almost every other upgrade needs it. It unlocks after you take over Whitechapel and lets you buy healing medicine.
Medicine Market is entry 8 of 9 in the Insider branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £500, 10 Metal, 50 Chemicals. Before Medicine Market unlocks you need taking over Whitechapel.
Black Market Ties
New weapons and other gear cost 20% less from shop inventories.
Delivery Service
More hanging barrels appear around London; shoot one to drop it on enemies below.
Dispatcher
Faster, nimbler fire trucks start reaching London's poorer boroughs.
Notoriety
The Frye name scares the Blighters enough that they stop starting fights with you in the street.