This answer is not specific to the BBC Micro, but is generally illustrative of editing technology on systems where document size is likely to exceed available memory. I'd allege that prior to virtual-memory systems, this style was the norm, since there was "never" as much memory as you needed.
I've edited fairly lengthy programs on a system where the core available to the editor (the 'amender') was only a handful or words larger than one disk block, as I recall 640 words or 3840 characters; this had to hold the input version of the file and the output version at the same time.
You can imagine two separate buffers for input and output, but space was tight. So the input text would be at the top of the buffer and the output at the bottom. You edit the text sequentially, a line at a time. As you proceed through the file, lines are transferred from the one to the other.
When you navigate off the end of the current input, the output is written out and the next block of input is read in.
There are of course complications in the amender for inserting more text than you've got space for, and perhaps if lines span block boundaries (I forget whether the latter was allowed, maybe there were no partial lines in blocks). But the user did not need to deal with it.
If you needed to 'go backwards' in the file then this can be accomplished by writing the rest of the output and starting again with that as the new input file.
So, with one simple restriction -- forward motion only -- you can edit in practically no memory at all (by modern standards).
A slightly more flexible editing arrangement puts the 'chunk' control into the hands of the user. Some amount of text is read into the available buffer space. The user can edit randomly within the buffer. When he's done with that buffer-full, he can move on to the next. This is sequential through the file, but randomly within a single buffer.
The venerable TECO, on many DEC systems, used this approach.
As usual, you can quickly accommodate working styles to the technological limits of the available tools. All of this was better than using a card punch.