Re: Transit UK from/to US
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.