My issue is indeed not overcharging, but rather lying about it. If they tell me that the price is the same in the city and make up some stories that there is a long waiting line and stuff, then I believe them.
I don't mind paying an airport premium, but then just tell me "here's your free SIM card and please pay a 10GBP service fee" or "the Oyster card is 3GBP but please pay us 7GBP service fee".
But they don't even mention that. On the receipt it's also printed "Three prepaid Simcard...10£".

Luckily I didn't buy the Oyster card because I was 100% sure that the oyster card wasn't 10GBP.

Contacting TFL or Trading councels is something I'm not really interested in as a tourist. I much more prefer to warn people over the internet rather than trying to solve the issue. Sorry about my selfishness.