The BBC site at Daventry used over 20 high power shortwave transmitters, each running about 250 kW of power.
It's highly probable that the aircraft's own internal wiring acted as an antenna at such frequencies, with sufficient signal level to become audible.
In the later 1970s, we were at Barksdale AFB and the crew chief would use a long lead to talk to us during pre-flight and start-up. Unfortunately this lead also picked up the local MF radio station.....a Southern bible-bashing station. So every time we crewed in we had to tolerate all the "Amen, brother" rants from some passionate Southern revivalist god-botherer - which made challenge and response checklist reading somewhat awkward.
Were any of you chaps on the Empire training scheme ever cornered by American bible-bashers?