Teletext used a system of control characters, which occupied character cells on the screen and were rendered as spaces. This allowed it to produce a 40 column 8 colour display of text and low-resolution graphics, with features like double height text, all while only consuming a fixed 40 bytes per line of data.
There were compromises in this of course, since control characters took up space on the screen you would often lose a character or two at the left edge of the screen, and while you could change the colour of a word, you couldn't change the colour of individual letters in that word.
The memory system in the BBC micro could fetch 40 or 80 bytes on each scan-line and feed them to the video hardware. This was deeply tied to the core architecture of the machine with the video reads being interleaved between the CPU memory accesses.
Assuming your characters are 8 pixels wide, 80 bytes per line in a bitmapped mode, gives you 80 colums in 2 colours, 40 columns in 4 colours, or 20 columns in 16 colours.
So you can't fully render a teletext screen in any of the BBC micro's bitmap modes. Either the resolution is too low, or the colour depth is too low.
While some paragraphs in the document suggest that reduced colours may be acceptable, other parts of the document clearly suggested that the BBC at least strongly preferred full teletext support.