I’ve heard about a “poor man’s copyright.” What is it? The practice of sending a copy of your own work to yourself is sometimes called a “poor man’s copyright.” There is no provision in the copyright law regarding any such type of protection, and it is not a substitute for registration.
From the UK Intellectual Property Office: [ipo.gov.uk...]
...a creator could send himself or herself a copy by special delivery post (which gives a clear date stamp on the envelope), leaving the envelope unopened on its return (ensuring you also know what is inside each envelope in case you do this more than once). Alternatively you could lodge your work with a bank or solicitor. It is important to note, that this does not prove that a work is original or created by you. But it may be useful to be able to show the court that the work was in your possession at a particular date.
In the USA the 'poor man's copyright' method just does not hold sway. It does not work. In the UK, there is no official copyright register. There are, however, many organisations acting as supposed 'copyright protection agencies', but they do not function in any official capacity.
The emphasis added in the quote makes the situation very clear that in the UK the envelope trick may prove useful, but there is no certainty whatsoever. The courts will not accept this as proof that you were the creator and thus the copyright holder. Copyright laws are far to complex for something so simple to be truly effective. Far better to have sufficient supporting documentation showing the creation and development of the work.