As I said on another thread, posting an envelope to yourself only proves that you posted an envelope to yourself. It could have been empty and not sealed and you could have placed the photograph (or whatever) in it a few minutes ago.
When I was advised to do this - many, many years ago in simpler times - by a lawyer, the procedure was to sign and date across both seals of the envelope, have a witness do the same, sellotape over said signatures and seals, and stick the stamps and registered-post labels across the seals of the envelope.
Edit: Should add, for clarity, that this does not imply that your work is now "registered" or now it is "copyright" and before you did it it isn't: There's no such thing as copyright registration in the UK and all work is subject to copyright. All it does mean is that IF you are in the unlikely event required to prove to a court that the work is yours this MAY provide some evidence that the work was in your possession before whoever else claims the copyright which MIGHT help your claim. Of course, the other person may claim you steamed off the stamps or whatever, but thats no different from you claiming they forged your signature on a contract - they're going to need some evidence to show you steamed the stamps and seals and sellotape, and managed to put it all back together perfectly... Remember - the burden of proof in civil law is only "balance of probabilities" so they need to convince the judge not that this is possible, but it's also plausible you'd have gone to all that trouble.
In practical terms, it's hard to see why this would ever be relevant in a photography dispute however: Photographers tend to hold the trump card: viz. "I've got the negative. I've also got the negatives for the 25 other shots from the same shoot. They haven't. On 'balance of probabilities', Game Over."
It might still provide a handy bit of protection for you before you post the lyrics for that Eurovision Song Contest winning hit song you've written off to Simon Cowell, though.
