I'm going to the UK tomorrow and I've only been there twice before. I'm a British citizen and they always refuse to stamp me, is there any way that I can get them to give me a passport stamp? Is there anything saying that they have to stamp me if I request it? I also have a German passport, could I maybe get a stamp if I used that?
There is no reason why they would put an entry stamp on a citizen's passport - you can come and go as you please, esp. within the EU. Using your German passport could be a problem - most dual-nationality countries require you to use their passport and not the other one(s).
Asia on the other hand is stamp-happy. My Japanese wife's passport has more in-out stamps from Japan than I have from all countries combined.
If someone needs proof of arrival/departure show them a copy of the boarding pass. Note that as a UK citizen you would have very, very few valid reasons to prove you landed in the UK on a certain date. However, if we turn the question around there are many reasons why you would need to prove you departed another country on a certain date. In that case an exit stamp would be better.