"The Film4 movie I want isn’t on any service." Solution: Region lock. Use a VPN to connect to UK servers (Film4 titles are most available via UK All 4’s app, but All 4 only allows rental downloads, not permanent). Alternatively, buy the region-free Blu-ray from Amazon UK.
Because Film4 is part of the Channel 4 network, you must use the Channel 4 streaming service to watch their films. film4 movies download better