Torch Computers packaged the BBC micro as a front end for either a Z80 (running CPN, later called MCP -- a ROM-based CP/M, IIRC) or, more interestingly, a 68000 which was my introduction to Unix. I only borrowed it for a few days -- didn't get much beyond the character-based Star Trek! No X server in those days, this was before Windows was around in any significant way.
Both these (certainly the Z80, which I still have) interfaced via the Tube.
Also AVRs are (arguably) more powerful than a 6502, and it took 2 hours to boot on a 16MHz AVR, so how long on a 1MHz 6502?
The BBC Micro had a switchable clock -- 2MHz for running code and ROM/RAM access, 1MHz for I/O (except IIRC the Tube, which was 2MHz). This was the "good old days" when everything (memory and peripherals) was accessed through a 16-bit address bus. The Beeb did have memory bank switching for the application ROM, to allow BASIC, DFS, NFS, and others to exist in the "same" 16K memory space.
Relevant to an ongoing Pi discussion -- both my Beebs had a fan added!
EDIT: On further thought, in the interests of historical accuracy, MCP was the 6502-side code, for disk access and maybe other I/O, while the Z80 CPN code was actually disk based -- either floppy or even 5/10/20MB hard disk. The Z80 had some bootstrap code in ROM, I don't remember having many issues with that. (My job included repairing Beebs and that generation of personal computing equipment.)