Actually you can in Germany with a Muttizettel (a note from your mum 🤣 - actually a signed letter from your legal guardian including their contact details and photocopy of their passport/ ID card). Not all hotels accept unaccompanied under 18s (more don't than do) but it isn't illegal. "Youths" are a category (14-18 year olds) separate from both children and adults who can under very specific circumstances live independently whilst remaining the legal and financial responsibility of their parents or legal guardian...
You can also provide a Muttizettel giving an 18 year old friend or sibling parental responsibility in specific situations, such as if the under 18 needs an x-ray or rather differently to go to a nightclub! (16-17 year olds are allowed in but have to leave at midnight like Cinderella 🤣).
Often if one of the party is over 18 there is no issue with the rest being under 18 when booking hotels, regardless of their being no blood relationships.
In many countries 15-17 year olds can stay in youth hostels and travel around without adult chaperones but can't book hotels but can potentially run into problems if something unexpected happens, like getting into an accident, when not having a legally responsible adult to give permission for treatment can create issues (not in life or death situations but more minor ones where someone needs to sign a permission form for treatment).
It is a good point though that in some countries under 18s are just regarded as children and social services equivalent can, should they choose which is unlikely, take responsibility for a minor without a guardian (unlikely unless they get into trouble legally or into an accident).
If you want to be on the safe side legally I suppose you should puck your destination countries carefully having researched each individual country's legal stance on the age group's status...
No idea why UK social services would be involved one way or the other though. It's not illegal in the UK for 15 year olds to be unaccompanied overnight.