Leaving Harrow in the morning, returning evening.

You will be going against the prevailing traffic, so it shouldn't be a
problem once you get to the A40.

...although sometimes Handy Cross (M40 junction 4) and the A404 can be a
bit of a pain.

I've often been held up in a two lane jam all the way up the hill to the
M40 from the Marlow junction. The OP should consider other ways to get
from Marlow to the M40, if possible! It might even be quicker to go down
to the M4 and east as far as J3, then north through Yeading and Northolt.

Why? That's absolute lunacy. The road up past Marlow Bottom (Wycombe Road)
is a very easy and hassle-free way to get up to Handy Cross. The longest
queue I've ever encountered going that way has been about 3 cars long, and
that's been with the A404 grinding to a halt just after the Marlow junction.
Of course, you could go up Sheepridge Lane (IIRC) and up through Flackwell
Heath, getting on the M40 at Junction 3.

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