Student tours of the U.K.
Student and youth tours of England, Scotland, and Wales
Most group tours are, frankly, full of people just to either side of retirement. Nothing wrong with middle-aged folk or senior citizens; just that their tours might not be the most fun for someone in their late teens or early 20s.
Enter the Student Tour, guided groups gallivanting around Europe where, for once, the age limit is a maximum and where the activities, destinations, itineraries, and evening entertainments are straight up a student's alley.
Some focus on fun; some focus on learning. Peruse many; find the one that best fits your style.