If you check out this link you may get some idea how you will get on getting round.
http://www.realtime-traffic.info/
The roadworks at Heathrow are mostly finished now and it is a fairly smooth ride, although I would take care to obey the speed signs if I were you.
The M1 backs up around junction 11 at Luton from around 5:30 in the morning southbound and with the roadworks that have just gone in, will be worse than ever. Stick with the A1 or A1/M11, it will be better.
The motorways around London are no worse than than those around Manchester or Birmingham outside the rush hours, its just that the rush hour lasts all day down here.