I only travel for leisure (visiting family/vacations) and most of that travel is short haul on low cost airlines. Luton Airport is the easiest one for me to get to, and I aim to be at the airport around 1 hour before the gate closes, travelling HBO - I absolutely hate having to check a bag in when visiting family as the flight is invariably to LIS and it takes a silly amount of time to recover it. This means getting a train into Luton about 2 hours beforehand. There isn't really a "plan B" other than buying a new ticket if the flight is missed (insurance wouldn't help as I am clearly not leaving the recommended time) so I may get stung one day. So far, so good! Only had a close call and that was due to the trains being disrupted on the main line - managed to get a lift to the airport at short notice and made it in with little time to spare.
However other airports are a completely different matter. Having to get a lift, or transit through London adds a degree of unpredictability. Stansted I'd happily be there an hour before the gate closes, but those taking me to the airport insist on being there at least 2 hours beforehand (being sensible and all). Gatwick I am trying for the first time late next month, staying the night before at a hotel as that was cheaper than peak time trains the next day (10am departure on a weekday).
I only use LHR for long haul, as it is a pain to get to - and as such I aim to be there 3 hours before gate closes. My partner's dad drives us to the airport and often we end up being there about 4 hours early, as they worry about the traffic and anything that could potentially go wrong. It's actually played a big role in me getting a lounge access card, as we travel in economy (I am too cheap
) and thought it would be nicer to sit in a different waiting room with some free food!