hmmm www.gov.uk/permission-take-child-abroad
"You automatically have parental responsibility if you’re the child’s mother, but you still need the permission of anyone else with parental responsibility before you take the child abroad."
so the tricky bit is actually proving you have parental permissoin from the father, if you don't have a letter. You don't have to have one by law but how else do you prove it? Bit of a grey area.
"A letter from the person with parental responsibility for the child is usually enough to show you’ve got permission to take them abroad. You might be asked for the letter at a UK or foreign border, or if there’s a dispute about taking a child abroad. The letter should include the other person’s contact details and details about the trip.
It also helps if you’ve:
evidence of your relationship with the child, eg a birth or adoption certificate
a divorce or marriage certificate, if you are a single parent but your family name is different from the child’s"
It's definitely not only been the UK security though, the Dutch security are jsut as careful. Letter worked fine though.
(actually in terms of grey areas, iirc if you are born to a british parent you are british but you don't need a british birth certificate. You're British whether you have one or not. The problem ofc is actually proving it! So in effect you need the birth certificate but technically you're British anyway).