Intransit Visa at London Heathrow airport

Hi All,

Im travelling to toronto on 4th sep, but there is a halt of 12 hrs at heathrow airport london. I have not been given a intransit visa(i hv no idea abt it)Can anybody give me information abt it. Can i get it directly at airport if i want to for a city ride? Or it needs to be carried from India?I heard that in singapore, they give it for 4 hrs 8 hrs etc....
The last option left for me is to stay inside airport