If you had entered through VWP(Visa waiver program) you can stay for a maximum of 90 days.
If you are coming to the United Kingdom as a general visitor you are allowed to stay for up to six months. This section explains the different types of general visitor and what you are allowed to do...
How long is the flight from Calgary YYC to London LHR? What is the mileage?
around 9-10 hours
7 hours and 30 minutes