The writer assumes that "you" refers to him or her. It's possible that the request not to write below that line is part of a document template. The instruction is for the template users not to put content below there, not necessarily to document consumers. Though the template could be used as a basis for letters that do have to be returned. Basically nobody in the entire workflow chain should put anything there, so that if that space has to be OCRed, it will reliably work.
> I am still not satisfied. If I send a letter back to TVL/BBC, and they scan the number at the bottom, it will generate the same information as they have already got; so, what's the point?
But that's the happy case, when nobody has written anything there to interfere with the OCR.
If the number cannot be read, then the document cannot be automatically associated with the residential address it pertains to; someone will have to deal with it manually.