If you are traveling with friends who are younger, the older members of the group (over 18s) should book the hotel rooms. A few hotels suggested younger members of the group should share with an 18 year old. (Of course, they won't really know which room you stay in!)
As 18 means you're an adult in the UK you can also drink in pubs and clubs. The 17 year olds might have a few problems getting into clubs so check before queuing. Have a great trip!
When looking for a great price for a London hotel I can strongly recommend the Hotwire Secret Hot Rate deals which I've tried and got an amazing bargain.