The US Military Intelligence Service had its headquarters in London at 37 Upper Brook Street.
The main training facility of the MIS was at Camp Ritchie, MD and trained men for 4 main specialties, and others. The OSS also had a role. The MIS was a War Department level organization.
Many of the instructors, and the training programs, were British (and Canadian) for all the various specialties.
My father was in the photo interpretation detachment of the MIS (our version of APIS at RAF Medmenham/Nuneham) and all his instructors were British and Canadian.
The HQ for this US detachment in London was 20 Ilchester Place. There was an APIS detachment working at Ilchester Place in concert with them.
We (the US) would not have had an effective photo intelligence capability in WWII without the help from the British and Canadians.