I absolutely hate suggested arrival times e.g. (For international flights, be at the airport 3 hours before departure).
Now I have never been outside of N.A., but on most flights, I have checked in online made it through bag drop in 5-15 minutes, then security in 5-10 mins. (30 mins max). The most I have spent trying to get to a gate was an hour in SFO, and that is because we had to go back through security.
Realistically, is 1-1.5 hours before hand enough time at Heathrow. It will be terminal 5, Monday late afternoon.