It's the law
http://www.ukba.homeoffice.gov.uk/customs-travel/Enteringtheuk/arrivingatukborder/child-travel-advice/

Since Section 55 of the Borders, Citizenship & Immigration Act 2009 - which specifically relates to the welfare and protection of minors - Immigration Officers are obliged to ensure that any minor arriving in the UK is either travelling with their own parents (or legal guardians) - or that there are appropriate travel arrangements in place for them if they are unaccompanied.

Yes, it's quite common for children to be born to parents who do not share the same surname - and yes, it's also common for them to travel solely with the parent whose name they do not share. Therefore we realise that in 99.9% of cases, the accompanying adult IS indeed the parent, and that there is no problem. However we still have to ask.

The point about carrying a birth certificate is a good one - even if it's just a photocopy. The full birth certificate does give both parents' names, therefore it provides a documentary link between the child and the parent. Without this documentary evidence, then it is always possible that the accompanying adult could be.. err... LYING when they say that they are the true parent.

Granted it's unlikely - but it's still not completely impossible. That's why a copy of the birth certificate is best, in order to avoid any hassle.

If the children are old enough to speak, and not too shy to communicate with other adults, then the Immigration Officer may ask the children directly to confirm their name and identify the parents. Otherwise, it becomes slightly tricky. And remember - all this time, the immigration queue is waiting, full of people who just want to get their luggage and go home.

The point about it being "hardly abduction if she's bringing them back" is incorrect: not everyone lives in the country of their nationality. You may be a British citizen living abroad, and abducting a child into the UK. Or maybe you do indeed live in the UK, but you have abducted them to live elsewhere in the UK, and you're simply returning from a foreign trip. The possibilities are endless.

So instead of complaining about UKBA officials for simply doing their job as instructed by the law, you should be thankful that the minor inconvenience of being advised to carry a birth certificate, to the 99.9% who have done nothing wrong, may well be protecting the interest of the tiny minority of children who HAVE indeed been taken without their parents' permission.
(EuroSong I love horse meat: best of all the animals, Fri 2 Mar 2012, 14:47, Reply)