As long as you have a valid US visa, a valid flight ticket and your transit in UK is less than 24 hours, you do not need a transit visa for UK. Off course, you will not be allowed to apply for a transit visa at the airport in UK so you have to spend your time inside the airport.
Please note, however, that immigration in some countries are not aware of this law and as a Pakistani you may get hassled because of that.
I dont know what are the rules, my friend from Pak on his first ever visit out of Pakistan, on his way to Canada, He asked the immigration officer at heathrow if he can 72 hour visa to see his friend in London, They agreed, changed his flight to next day and He phoned me up, He didnt have any visa for UK got it from Airport here. All he had was landing papers to Canada.
It was nice for him and i shown him around london for couple of days. and it was about 9 years ago.