Fallout: London devs hand out maths homework to players who are suffering from a bug that lowers their HP

pcgamer.com:

Fallout: London's release has not been smooth sailing. After starting with a confusing installation process and dealing with modders opting for the word soccer over any references to football, many players are now facing a frustrating bug that messes with their HP. 

This debuff is linked to a game-crashing glitch that affects players who ride a train through apocalyptic London in the mod's opening hours. Now aptly labelled the «train crash,» this bug has a lasting effect on players' health and, even worse, could be stopping them from booting the game entirely.

«If you're getting one-tapped by a fox etc, this is probably why,» lead writer for Fallout: London Callum Quick says in a Discord message. «There's an error with how the player's health is handled after the train crash, which disproportionately lowers the max HP of characters with already high health or endurance levels. This will be fixed in the first update, and the effect of the „Lab Rat“ affliction will be increased so the same sense of desperation is felt early-game to an extent.»

But if you can't wait for a new update to fix everything, then Quick does include another manual way to restore your health back to normal: «For your current saves, work out what your character's max HP should be using the formula below, work out the difference between that and your current max HP, then use the modav command to get it to where it should be (do not use setav) Or, if you like the challenge, leave it be!» 

You should also see if you have the «Life Giver» perk, as this will slightly change the formula. If you're on survival, make sure you briefly drop down to hard or normal mode so you can access the console. Or if you don't want to mess with your survival run, there's a mod that'll let you access the console on survival mode. But honestly, it may just be easier to take the L and live a life in fear of rabid foxes because when I tried working through the equation myself, I was halted by horror flashbacks of GCSE