Seemingly predating that StarDot thread by a few years, I found this [list of documents][1] that includes "HardwareSpecification.txt", "BASICOutline.txt" and a few others that are "(c) Copyright BBC 1981" rather than copyright Acorn, or dated 1982 when the BBC Micro actually appeared. There are various details not from the StarDot thread, including:

 - unused gate inputs will not be left open-circuit;
 - with the sole exception of the Econet interface, integrated circuit sockets will be fitted in all unequipped positions;
 - dimensions: 400mm wide by 300 mm deep by 60mm high approx;
 - the case will be of two-part construction, the upper shell supporting the keyboard PCB and the lower shell the main PCB and power supply. Electrical connection between the two parts will be by flexible ribbon cable;
 - [it is] essential that all points which could be at mains potential when power is applied be inaccessible to the "standard finger";
 - it must be possible to generate all ASCII codes (0/0 to 7/15) by using the SHIFT and CTRL keys in conjunction with the other keys;
 - a RESET (or BREAK) key will not be provided on the main keyboard;
 - both pound sign and number symbol (hash) must be included ... the RETURN key will be a different colour from the rest;
 - the power supply will withstand an overload indefinitely and will protect itself from damage through overheating, even at 260v mains. The power supply will NOT be a switched-mode type;
 - a 1v pk-pk (75 ohms) composite video (PAL coded) output will be provided on a 75 ohm BNC socket accessible at the rear of the machine. The socket will not be fitted in the basic machine;
 - a cassette modem will be incorporated ... the demodulator will be insensitive to input level variations of up to +6dB or -12dB and must recover the UAR/T clock from the tape in order to track short and long-term speed variations, It must cater for an instantaneous speed error of at least 10% WITHOUT relying on the inherent insensitivity to speed of asynchronous data, i.e. bit-centre sampling must be maintained;
 - a parallel printer output to Centronics specifications will be provided. The 6522 I/O device will be fitted in the basic machine but the buffer components and connector need not be fitted as standard although provision will be made for these on the main PCB;
 - provision will be made on the main PCB to fit a floppy disk controller plus data separator and buffer devices to allow interfacing to one or two mini-floppy or 8" floppy drives ... both hardware and software must be capable of supporting 8" disks to the IBM 3740 specification, although this may necessitate fitting an additional ROM which is not present in the basic machine. There is no requirement for double-density operation;
 - an elapsed-time clock will be included in the basic machine, having a resolution of 10ms. The clock can be set and tested under software control;
 - there will be 8 selectable display formats as follows ... (followed by exactly the BBC Micro's modes, including the requirement that 'text' modes be bitmaps);
 - in modes 0 to 6 the "colours" are selectable from a palette of 16 effects being black, red, green, yellow, blue, magenta, cyan, white and the same eight colours automatically flashing;
 - the CPU is a 6502A running at a 2MHz clock rate except when accessing some input-output devices, when the effective clock frequency is reduced to 1 MHz; and
 - the level of R.F. radiation from the machine is to be minimised. The exact maximum level is to be agreed by the BBC.

It sounds to me like that document might be after Acorn has become the de facto supplier, given the specificity of the processor, the display modes, the implementation of econet, etc, but I'm pretty sure that even the Model A came with a composite BNC and that 8" disks were never supported, and certain that the return key ended up being the same colour as all the other keys and that break is not only present but shift+break is the intended way of launching almost all disk-based software. Various other specifications I haven't quoted also weren't eventually met. There's also still quite a lot like that final item that is reserved for later specification by or agreement with the BBC.

So this is a statement of specifications intended to be met, not a description of the machine as completed.

  [1]: http://chrisacorns.computinghistory.org.uk/docs/BBC/