You don't need to send the original passport as there's a risk of losing it. The ECO/ECM only needs a photocopy of your passport and the stamped pages, and you can ask for it to be signed by a solicitor to certify it's a true copy. All the original documents that you have must also be photocopied, so your fiancee needs to submit 2 sets of documents, originals and photocopy. VFS keeps all the photocopy and would return all the originals.