Originally Posted by corporate-wage-slave

Fot Guernsey, you really don't need to worry, I'm assuming he is under 16 (given that in the past I've typed reams of screed in a previous post, where it turned out that said child was 32 years old). By all means get that bus pass, ready, a school or library card, and a birth certificate. If he is named on that certificate and at least one parent is on the certificate then there is a clear link to someone else, an adult, with ID. The chances of any of this being needed will be well below 1%.

For Mexico. I don't see BAH being flexible beyond the unrelated policy mentioned by Tobias above. Have you Followed the Passport @HM_Passport on Twitter and then sent a Direct Message giving the details? Realistically you may want to consider buying a cheap Ryannair flight to mainland Europe, which you may not use, to leverage the 7 day or Premium service. Note also the Twitter bot that checks when these in person slots are opened up. The suggestion to contact your MP is also a good one, all MPs offices are getting used to escalating passport issues.