Andrew Wyatt wrote:
Opinions requested - in two weeks time on a Thursday my family will be
coming to visit us, and will be driving from Heathrow Airport to
Walthamstow, starting out from Heathrow at about 15:30.

I guess the two obvious routes are M25-M11 or A4-A406-A10. What do you think
would be quickest at that time of day? Typically, how busy are the two roads
main roads (M25/A406) then?

A10 would be the wrong exit from the A406. For Walthamstow you will
want to exit on A112 (Chingford Road) at Crooked Billet or possibly
A104 then right onto A503.

Also you'd want to take the route A312/A40/A406 rather than A4/A406.

The parts of the A406 you may find slow are around Henlys Corner (A1
junction) (not usually so bad in that direction, worse the other way)
and Bounds Green to Palmers Green. There are some road-works at the
moment just before Henlys Corner but they are night-time only and
shouldn't affect you.

The M25 route is a long way round. Might be worthwhile though. As an
alternative to leaving at M11 you can leave at J24 and take the A111
until it meets the North Circular at the Great Cambridge Roundabout.
That road is generally fairly clear except for a small hold-up in the
middle of Southgate.

Then there is the "central" route:

A40 all the way to Marylebone (where it becomes A501)

left onto A4201 (Albany Street) (just after Regents Park, which you can
go through as an alternative). Follow the road round into Parkway and
at Britannia Junction (Camden Town) take A503 Camden Road and follow
this road all the way to Walthamstow.