Apparently the UK govt has given the green light for this, as the list is only of countries that the government has allowed armed US customs officers on their territory, and, the airports are ones that have bidded for the USPBC and the DHS has deemed the airports suitable to house the facility.
It seems its just down the the airport and airlines to figure out how they are going to do it and pay for it, but, as MAN put in an application one can only assume they are happy to pay and have a plan.