Is it possible to get a visa for 2 UK citizens who want to travel to Lhasa from Kathmandu and Xi'an?

It's possible for you to apply for a China visa for your trip to Xian, but you'd better not mention that you will go to Lhasa as well. You'd better get the visa in Uk if you are are Uk pasport holders. After you enter China, you can apply for a a Tibet entry permit through taking part in a Tibet tour by a China travel company.

However, if Lhasa is your first stop, the thing will become very complicated. You will have to take part in a China tour group and fill lhasa in your China visa application form when applying for the visa in UK. Once you get to Kathmandu airport, your previous China visa will be cancelled, and you have to apply for a new China visa with your Tibet entry permit.