Bideshi...u dun need to get a visa stamped on ur passport for a transit at heathrow...last time I went to nepal i had a transit of 5 hrs at heathrow as well.. so dun worry...n btw it doesnot matter whether u have H1 or F1 or B1/B2....u dun need a visa as long as u stay inside the airport terminal.....happy journey bideshi..