I never saw the BBC machine. DeSmet (Aztec) C for the Apple was full ANSI in its final incarnations.
The //gs system software is available free here:
The APW tools seem to have disappeared from Apple's site - they were there in December 2010 when I made the previous post. They APW tools were based on ByteWorks ORCA tools which are still available.
An early version of APW is available free from:
The ORCA environment and compilers are available for $$$ (though reasonable) from
I originally found the APW stuff googling 65816 developer tools, so it's possible they are mirrored somewhere ... but I can't find them again just now.
There are free 65816 C and Pascal compilers. You can get the ByteWorks C and Pascal GS/OS and //gs toolbox interfaces from Syndicomm and cobble together a developer system.
APW didn't have an APDA sanctioned Pascal. ORCA/Pascal was the plug-in Pascal for APW. Overall ORCA was a better compiler, but TML's integrated debugger made it a nicer development environment.
TML (now Complete Pascal) for //gs is available here:
George