The system sucks and is completely irrational and completely on the whim of which ever immigration official is on for the day. Some are sensible others are complete cocks!
I travel alot and did when Ex and I were together - never once got asked.
Now it is constant and the wanker on the desk can make up the rules on the day, i have been asked for:
birth cert - had it
then wanker asked for my marriage cert - did not have it - sos gave me a lecture about unmarriede mothers, culture
Another time birth cert, marriage cert, proof of address, lecture on my morals, his right to take my children away from me, for their safety etc etc.
Eldest DC is now old enough and not just interrupts and says - why don't you ask me, she's my mum - do you have a problem with it. Last year wanker then tried to tell him he was protecting him!! DC replied yes but you enver ask my Dad and I do not live with him, he is not British and his family live overseas.!!!!
What pisses me off they can see on the screen , I left the country with them and am now bringing them back into the country and having been doing that for years.
Either make it compulsory but do not say it is voluntary and then berate people.
More kids are abducted by their father, who have the same surname but don't worry immigration will not question them because having the same surname means you are less likely to abduct a child!!!!!
Apologies for rant seriously sore subject in my household.