Agreement (Exequatur): After the UK announces the appointment, the receiving country (in this case, the U.S.) must formally accept the individual. This is called the exequatur process, which involves the host country granting the ambassador official approval to serve.
Apparently....through quiet diplomatic channels.......the US should have already been given the nod as to who it is going to be......and maybe accepts unofficially.
Maybe the US said no.....and the UK is playing political games announcing it this way.