Upvote:7
If the connection or one leg is cancelled, the entire itinerary is cancelled or rebooked. They will NOT fly you to Heathrow, if your connection is 3 days later.
The only exception would be if the cancellation is out of the blue, i.e. not know when you board your flight in Sri Lanka. This is exceedingly rare and most airport/countries of emergency procedures for that. Typically you would get a short term emergency visa or they put you up in an accomodation with no access to the country.