Payingpax, If the flight is on a Saturday or on a holiday, absolutely check in 3 hours before departure, Heathrow is a madhouse on the weekend. If your flight departs at an off hour,you can let it slide to 2 hours before. Please bear in mind if you do not have a reserved seat if you check in at the last minute you will be left with a less than desirable seat, or not seated with your family ( not always a bad thing).