This depends on whether you fly direct from London to Colombo or take a transit in Another Country. Currently (as at 10.06.11) there is only one air line that flies direct from London to Colombo and that is the national air line of Sri Lanka (Sri Lankan Air Lines). Direct Flight takes about 9.30 to 10.00 hours.
Majority of the transits are in the Middle East. The transit time may vary depending on the air line. I have flown few times across Abu Dhabi and it is usually about 2-4 hours. Dubai will be about 3-4 hours.
Unless someone has a time constraint, I would advise to fly via middle east as the direct flight is too long.
11 hours
London Heathrow (LHR) to Colombo (CMB) by a one stop flight operated by Srilankan Airlines would take about 12hr 55min. The flight stops at Male (MLE).
10hr 45min London Heathrow (LHR) to Colombo (CMB) by a nonstop flight operated by Srilankan Airlines.
It takes about 1 hour by road to get to the center of London from Heathrow.
How long flight from London to Karrachi
From London Heathrow to Cancun, Mexico flight time around 8 hours
It takes about eight hours to get from London Heathrow to Uganda.
It takes about 8 hours flight from islamabad to London heathrow.
about an hour and a half
About 2 to 3 hours.
Departure point: London, United KingdomDestination point: Democratic Socialist Republic of Sri LankaEstimated flight duration: 10 hours, 46 minutes
9 Hours kind of