Stimpacks as one of the healing methods in Fallout London. You can heal your character, but I personally use to keep my companion Churchill the Dog.
In Fallout: London, you can find and purchase stimpaks and other medical supplies at various locations throughout the game.
Where To Buy Stimpaks In Fallout London
Thameshaven Market has shops and vendors where you can buy stimpaks. The market is unlocked during the Charybdis quest after you help The Ferryman.
At the market, speak with Leana Searle and Dr. Alan Webb to purchase and trade Stimpaks at their clinics.
A single Stimpak will cost you 147 Tickets, but you can unlock perks that will improve your bartering abilities. You can use bartering to lower the prices.
I also managed to team Stimpacks from Dr. Alan Webb’s stand so you can also do that but make sure you have good stealing skills.
Note: If you have ailments such as Radiation or simply want to restore your HP, Dr. Alan Webb (and other Fallout London doctors) can cure you. Radiation Cure is 80 Tickets and it only costs 30 Tickets to restore full HP.
The Swan and Mitre is another location that has a trader named “Sabertooth” Syd, who sells various goods, including stimpaks.
Besides purchasing and stealing, you can also find stimpaks in various locations as loot. Try exploring abandoned buildings, raiding enemy camps, and completing quests can yield medical supplies.