It’s not true that PreCheck does nothing outside the USA. On all USA bound flights a percentage of passengers are required to be swabbed for explosives and have their electronics checked (done at the gate at LHR), and if the flight is on a PreCheck participating airline, this group of selected pax will come from the non-PreCheck passengers. So yes departing LHR-USA on a PreCheck participating airline with your KTN in the booking will bring you a small benefit, not at regular security but in avoiding a gate swabbing.