There has been some discussion about this on the SOG mailing list
conclusion was
"I spoke by phone with the SoG this morning. Unsurprisingly, they are short
of space and short of staff. However, my phone call was most productive. It
is clear that, at present, the SoG doesn't have space to accept everything
so, our cunning plan was for me to document a list of my old original
documents which (no guarantee) they might try to find space for. That is an
offer I was happy to accept.
Meanwhile, I will also digitise everything else (e.g. old letters from the
1930s). The SoG will accept the digitised version and, once they see the
content, can decide whether they would also like the paper copies (e.g.
correspondence from the 1930s).
I will be happy to be guided by their advice."
and
" a useful page on the SoG website. For those who
haven't seen it, it provides guidelines on how to prepare ones papers for
the SoG."
http://www.sog.org.uk/learn/share-your-knowledge/