Edit: SOLVED! See my later post for the solution.

I just noticed a weird thing: My ranged weapons are doing half damage.
I might have picked up a bugged item or perk, because the damage numbers are down to half in the weapon descriptions and I'm having a harder time killing stuff as well. For example, I had two sniper rifles (service rifle and crude sniper rifle), each doing around 40 points of damage and suddenly they are down to around 20 points of damage.
Any ideas?

Degradation of the weapons?

https://www.gog.com/forum/fallout_london/after_reading_combat_coaster_card_all_weapon_damage_gets_displayed_as_halfed_in_my_inventory

I already have also done a topic about that. It´s the Card "Combat Coaster" in "The Hound" pub with the double headed dog on it.
After realizing what happened i stepped back through all savegames and finally found it - lost about 2 hours with that as i started playing from that point before picking up that card. No idea yet how to remove the perk from it with the console, so i can only suggest that you also load a savegame previous to picking it up.

Thanks, mate! Now I just got to find out how to remove that via console or I will lose many hours of exploration. :P

wait, what?! This is why it's been like swimming uphill all the time!? Haha, and here I started to think the extra ammo finding perk seemed mandatory.

My save is like, 8-10 hours after the Hound Pub, probably going to need some console commands for this.

Inside the Fallout London installation folder is a data subfolder, inside this folder are 2 .txt files named folon_sperks - both filled with id´s but no information what does what. I did not fully try it yet but i bet one of those in these files is the related one.

SOLUTION
You can remove the coster and the perk via console and once you select a weapon in the inventory its damage resets to full.

To remove the coaster:
player.removeitem 0727D392

To remove the perk "Combat Beer Mat"
player.removeperk 0727E475

Once the perk has been fixed you can re-add the coaster:
player.additem 0727D392

If the IDs dont match or you are looking for another bugged perk: Use "help X 0" where X is the word you are searching for to search the in-game database for IDs.

Thanks to LirdNighthawk for putting me on the right track!