United Kingdom Transit visa when traveling to the US
Nepali passports holders will not need a UK transit visa at the London airport if they have a valid visa in their passport for travel to the United States or Canada. The rules state that a valid visa from the US or Canada must be stamped in the passenger’s passport. Permanent residents with a green card are allowed to travel via London for transit.
On the other hand, folks who may have proper travel documents through asylum, or are in the process of getting a green card, will not be allowed to board a flight via the UK without a transit visa. These documents are a separate legal document that are recognized by the US for reentry, but not recognized by the British authorities for transit purposes.