What time you leave home is all but irrelevant, it’s what time you hoon down the M1 or M11 or whichever road(s) you take to arrive (and at what time) on the M25 that is relevant, as it might dictate what the traffic might be like.
I drive on the M25 quite often, either anti-clockwise from London E1 to the M1 or M40 or M11 or A1 or clockwise to the QE2 crossing at Dartford or on a short hop from the A2 to the M20. It is not so full of horrors as people dream it up to be. Yes, there can be hold-ups. Yes, there can be a queues at QE2 bridge or the tunnel but outside of the rush hours, things are usually reasonably free flowing.
For a long while, the the M25 has had long 50 mph speed restrictions, whilst the apparently tested something or other. These are no great inconvenience. Whether the restrictions have been lifted in the past month or so, I don’t know,
My advice? Toss a coin or better still, ask Google when you leave home. That though will give you its best shot at 07:00 AM…. Things might change…but that’s Adventure (with a capital A) biking for you…. Life on a knife edge in the 21st century.
Enjoy your holiday.