Probably about 4.30 to 4.45 am to the carpark. It always takes longer than you think to find a space, wait for the bus and get to the terminal.
Then 5 am is quite a busy time at airports because lots of flights leave at 6-7 am or so, so security queues might be long. Also think about whether you will want to get breakfast. It all takes time. Aiming for 4.30 am or so, also gives you extra time in case of breakdown or traffic jams.
Last week It took me 1.5 hours to drive less than 50 miles to the airport, despite setting off at 5.30 am and the journey being at least 48 miles on the motorway, simply due to queues.
I do the same journey and flight a few times a year and luckily this time it 'only' took half an hour to get through security. Sometimes it takes over an hour because too many people still don't understand the liquids rules and if it had taken that amount of time this time, I would probably have missed my flight.
The other consideration is what the consequences of missing your flight are - if you have to rebook on another flight the next day, and there's more than one of you, it could cost hundreds of pounds or more, and your travel insurance probably won't help you, if you didn't allow sufficient time for possible delays.