RISC OS, the ARM descendent of the BBC Micro OS, runs on the Raspberry Pi, and there is even a "Pico" version that boots directly into BBC BASIC (though it is now called ARM BASIC V):

https://www.riscosopen.org/content/downloads/raspberry-pi

It is the closest you can get to a BBC Micro today, and it is extremely close. A BASIC library for working with the GPIO pins is even included.

The whole thing is Apache 2.0 licensed.