This may seem to be a strange question.
I would like to have a proof that shows I enter the U.K. on June XXst and leave the U.K. on August XXst, or anything of this kind.
Generally, the stamps on my passport will do.
Immigration officers in other countries always leave two stamps on my passport (one for entering and the other for leaving), so I can just use my passport as a proof of where I had been.
However, the U.K. is one of the countries that do not give you a leaving stamp, so there is only one stamp on my passport, indicating the date I entered the U.K.
So, now, my question is, what should I do, if I would like to get a stamp or a document or anything that shows the date of me leaving the U.K.?