This is really tricky to make your parents understand such things. At the same time, they do understand you but their concern for their child is right. if you want to go on solo trips, make your parents believe that you are not into such things and also do let them know about your friend circle, give them every little detail about your trip and where you will visit in London.
Keep your parents updated about your daily life and make them trust you for this trip. This is the only way you can make your parents understand that you will not get involved in any bad influences.