I could well be talking out of my yin-yang here, because I've been out of UK for a couple of years, but I found the most traffic-free route (from the M6/M1 junction) to be A14, past Kettering (very fast dual carriageway). Ignore temptation to go south on A1 - keep heading east towards Huntingdon. (Beyond A1 turn-off there used to be several speed cameras, so ease-off a bit in this section (St Neots?).
You end up on M11 south (nothing like as busy as M1 and A1 on the approach to M25) Then onto M25 towards Dartford.
This route bypasses one of the busiest sections of the M25 and the A14 and M11 allow you to get your boot down a bit. A little caution should be excercised on the approach to M25 as Essex Constabulary are reknowned twats, but characteristic of the M11, you can see 'em coming.
I might be on crack, but this used to be a really trouble-free drive.