There's only two real options when one country annexes another
- You (grudgingly) accept it. You might wring your hands and rattle sabers (i.e. sanctions), but do nothing else of substance (if you do anything at all).
- You go to war and force the invading country to cede the territory (and, of course, there's that little issue of keeping them out after you're gone).
Tibet isn't terribly important country to most folks (it's still a mostly agrarian society), and China has a very capable military. Add in that the history of Tibet is sometimes under Chinese rule and sovereignty claims gets murky fairly quick. War with China over a fairly mountainous and land-locked country would be difficult and costly on multiple fronts. As such, most countries don't hold an official position. The closest the US has come, for instance, is in applying pressure for greater access to the region
The law seeks to promote access to Tibet for U.S. diplomats and other officials, journalists and other citizens by denying U.S. entry for Chinese officials deemed responsible for restricting access to Tibet.
Which of course made China unhappy (which was probably the point)
“If the United States implements this law, it will cause serious harm to China-U.S. relations and to the cooperation in important areas between the two countries,” Hua said.
The official UK page has lots of hand wringing, punctuated by this
The UK’s policy on Tibet remains unchanged. The UK recognises Tibet as part of the People’s Republic of China and does not support Tibetan independence. During 2014, we continued to call for all parties to engage in substantive dialogue. We pressed the Chinese authorities to exercise restraint, respect religious and cultural freedoms, and allow unrestricted access to Tibetan areas for international journalists, NGOs and diplomats.
Which, if you'll note, is more or less the same position as the US. I can't find any country where the government openly supports the independence of Tibet. Taiwan is in more or less the same boat.