Goonhilly / Telstar
A former lecturer of mine at college in the 1980s once told me that the very first US to UK transmission involved a US generator and a US TV set hooked up to the incoming signal, and a UK TV camera pointed at said TV set, whose brightness was fully turned up to get enough persistence on the phosphors for the camera to even /see/ anything.
I believe he had been a GPO engineer during the period.