Lee wrote:

Used to be suggested that sending it registered was deemed sufficient
proof, since the PO would check it was sealed and stamp over the
"openings".
Don't know if it's still the case.

To prove a date on something, I think you need to lodge it (or possibly
have it sealed) by someone reputable, I wouldn't include the PO in that
category.

I recently had a registered delivery disappear back into the system
after PO initial delivery attempt, their "please collect" postcard was
the only evidence I had that it even existed, for a week they didn't
know where it was, eventually they did phone to say it had re-appeared,
I went to collect it, just in case ...