I can help a little here as I work for a local council, currently administering an all-out postal vote. Incidentally, we would not have chosen to run a postal vote as the old system is much easier.

I disagree about it being not secret. Here is what our elections officer has said about the barcoding which seems to be the source of a lot of angst:

"The barcodes are on the back of the ballot papers as a security measure. The barcode is used to scan, through the window of the envelope, the unopened ballot papers back into our software. If we were then to receive a fraudulent ballot paper e.g. a photocopy, the software would inform us that we had already received a ballot paper with that barcode and an investigation would then take place.
I can assure all electors that their vote is anonymous. When the envelope is opened, in secure conditions, the declaration with the voter's signature is separated from the smaller envelope which contains the ballot papers. The declarations are removed and stored separately before the ballot paper envelopes are opened. Once the ballot papers are removed they are stored in secure accommodation in sealed ballot boxes until the count."

Cheers
Sarah