when you plan to do adventure tours to Lhasa, the capital of Tibet A.R., you are still required to hire a local guide, and the guide will be go with you to reach Lhasa. This is the official regulation from Xizang tourism bureau, Xizang Armed forces and Xizang Millitary Managing headquater.
Taking overland trip to Lhasa, you can start your trip from such mentioned cities or towns as Chengdu (capital of Sichuan province), Kunming (capital of Yunnan province), Xining (capital of Qinghai province), Lanzhou (capital of Gansu province), Lijiang ( a major tourism city in Yunnan province), Kashi ( a small town in far western tip of Xinjiang Uygur A.R.) and Kathmandu (capital of Nepal).
Guide service is only required when you are inside the administrative boundary of Xizang (Tibet A.R.); this mean before you reach the boundary of Tibet A.R., you do not need to have guide service; during the time, you can travel around freely (with the escort of local guide), and you cna only hire a english speaking driver, that is enough and can help to save your expense.