I dont think any caravan sites with lots of facilities rent out the caravans to groups of young people.
Have a look on Haven website and see if any places suit.It will say if they only let to families and couples.

I've looked on the Haven website and there isn't much anyone is interested in.
Caravan placed generally do let you in if there is someone who is 21 with you, I just can't think of any places that everyone will be happy with.

Hello! I want to travel in the caravan in the UK this summer. Does anyone know how to arrange this? I want to hear reviews from already experienced ones) By the way, do caravans have a postcode? can I order something there at night, for example? Does anyone use this https://postcodefinder.net/england/leeds to find out the postcode? I hope to get a prompt reply)