The reason short cab journeys are so expensive at Heathrow are due to three things:

1) There are a lot of fees that Heathrow add on
2) It isn’t 12 mins work, before that there is a massive queue, for both black cabs (real queue) and Ubers (virtual queue) to get a fare, so you effectively need to add a contribution to the time they spent waiting in the queue - they don’t earn while they are waiting
3) There is the opportunity cost too, most fares around Heathrow are £50+ due to the distance, and many are £100+ It must be quite gutting to a taxi drive to wait 40 minutes in a queue for a fare, then only get 12 minutes of work, and find themselves still at Heathrow, so need to get back in the queue for more work