https://www.riscosopen.org/content/downloads/raspberry-pi
They used to offer a BBC BASIC-only image called RISC OS Pico, which you can still find on the Internet Archive and should still work on older Pis:
https://web.archive.org/web/20181109020203/https://www.risco...
https://web.archive.org/web/20181109020203/http://packages.r...