I seem to recall getting to Heathrow T5 very early in the morning a dew times, a few years ago for work purposes. Security seemed barely awake pre-6am, so despite very few passengers at that time, it still took a while to get through. You don't state from which direction you will be approaching T5, since that can make a huge difference especially in the early morning.
As chance would have it, I will be on the last scheduled flight of the day to Berlin on Friday night; am driving to the airport and have checked baggage. I intent to arrive about 2 hours before the publicised departure time, and you never know what the M25 will be doing on a Friday night (I intend approaching via the M3 and M25) - I always have a back-up plan or two to take alternative routes in case of traffic issues. I have previously cut across country via Chertsey/Ottershaw and joined the M25 at Chersey to avoid the M3, can also go via the A30 instead of the M3 or go via the Terminal 4 exit and perimiter road...... if timing is critical you should work out some alternative routes beforehand. This may include heading to Hatton Cross tube station and taking the underground into the airport if traffic is really bad.