I am working on an improved version so you can curve your draws and get more angles
I thought the essence of Turtle Graphics was that everything is relative, in other words you can turn (say) 90° clockwise without knowing in which direction you are currently pointing. So if you write some turtle code to draw an object, you can draw that same object in any orientation you like just by setting your initial direction.
Your code, on the other hand, seems to use absolute directions all the time. I'm no expert in the subject, but I fear that throws away many of the advantages of turtle-style graphics. I wonder why you didn't simply copy, for example, the Liberty BASIC way of doing it: it has commands GO for moving forwards (in whatever direction you are currently pointing) and TURN for changing your direction. It also has HOME and NORTH to establish an absolute reference.
You can even find BBC BASIC code for these commands in lblib.bbcc (the Liberty BASIC emulation library) so implementing them involves very little work!