I usually aim for 2-2.5 hours before and I've never had a problem, though I appreciate that this is probably just luck on my part (though I do fly a lot). I think the biggest difference is whether you are checking a bag or not. If you just have carryon, and can either print out your boarding pass before hand or use a self-checkin machine at the terminal, this saves tons of time.
It also depends how you are getting to the airport. If you are taking tube or bus all the way, you may want to leave more time but the Heathrow Express/Connect are pretty reliable. But you may want to leave a bit earlier anyway because 8 am is full rush hour swing, you may find yourself saving loads of time and discomfort just by leaving 20-30 minutes earlier.
My motto for heathrow is "expect the worst" but generally departures haven't been too bad for me; I do have the worst luck with arrivals though, as quite a few times I've ended up sitting on the tarmac for a long time for no reason, only to find an endless immigration queue. Yuck.