What permissions does my 15 year old need to travel to Spain with a friend's family?

[SuSu22]

Hello,
I have only just realised that we need to consider this. I was too wrapped up in GCSEs. Our 15 year old is flying to Spain with a friend's family. I imagine we need some documentation to show we agree to this. I am hoping a birth certificate isn't required as I don't know where his is due to a house renovation. TIA for any advice from those who have done it.

I googled when we were taking our daughter's friend with us on holiday and there was a standard form you could get signed by both parents giving permission. We weren't asked at any passport control (UK or Greece) for these documents, but had them in case. She was 15 too.

Something like this

Parental Permission

We took DD friend to France with us and printed a form off the internet which her parents signed. We had to put in passport details etc. She was not asked for it at passport control.