[KerrAvon]

Some good discussion of that on The BBC BASIC wiki entry: https://en.wikipedia.org/wiki/BBC_BASIC (paragraph beginning "Due to a number of optimizations[…]".

[crimbles]

Also you could swap out hot spots with assembly trivially as it had an inline assembler. And you had indirect pointers in it!

[mavhc]

The OP's code has REM based assembly it in to save space

[kragen]

is that assembly or is that the dithering table

[alphabetter]

Yes, the REM is the lookup table used in line 70.

Because BBC Basic had a built-in assembler it was pretty uncommon for BBC programs to inline machine code as raw data (unlike some other computers from BITD).

[londons_explore]

just a table of constants.