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.
and 911 happened 8 years ago.