Rather neatly, you could chuck a second CPU on the end of the bus and just use the host machine for IO. That made the machines EXTREMELY fast for the time. Many second CPUs were developed with different architectures from 6502, Z80, 32016 and the original ARM CPU (which was developed as a slave of a BBC micro!). I myself have owned a 6502 unit.
You can still get NEW tube second processors including a 64MHz ARM7 with 64MB RAM ( http://www.sprow.co.uk/bbc/armcopro.htm )
Pretty awesome for a 1980s 8-bit computer eh?
More info: http://en.wikipedia.org/wiki/Tube_%28BBC_Micro%29