Many years ago when travelling back from France with my uncle (different surname), we were refused travel on the Eurotunnel as I didn't have a 'permission letter' from my parents for him to take me out of the country (we were trying to come back in...). When my uncle questioned this, stating that he himself could write the letter, the gentlemen just shrugged his shoulders and rang my parents to 'get permission'.
While I understand they're doing their job, and quite rightly too, the system seems very flawed.