Churchill Mining appears to be a British corporation. Not that that clears up your question any. But Dersu Uzala was made at a point where Kurosawa’s career was basically falling apart, and he could no longer secure any domestic funding to make movies, which is how he ended up working for a Soviet film company in the first place. Possibly, Churchill Mining got the rights to the film as part of a deal to help fund it. Stranger arrangements have been made in Hollywood.
Although the British “Churchill Mining Company” appears to have only been founded in the mid-2000’s, apparently mostly to pursue some iffy mining claims in Indonesia. There are some references to a “Churchill Coal Corporation” that was apparently a mostly phony coal company set up as some sort of tax dodge in late-70’s Pennsylvania. Perhaps there was some sort of tax advantage to purchasing the rights to obscure foreign films.
FWIW, one of the other two production companies I see listed are SATRA, which was one of the handful of US companies allowed to do business in the USSR at the time. They mostly dealt with ores and such, but apparently had a sideline with films going both ways. The other one was New World Pictures which was Roger Corman’s production and distribution company, which apparently dealt with the odd foreign movie to try to distance itself from its usual schlocky B-movie fare.
Here’s the list of distributors in various countries that the IMDb gives:
Here’s a long document about fraud in the coal industry. It mentions the Churchill Coal Corporation. They apparently set up tax shelters in businesses like coal and movies: