The Metropolitan line is quite convenient...
I have occasionally done that section on a bike, including at (roughly) commuting time. I believe I went straight down the A404 before turning left quite close to the Westway to pick up the A5 for the last bit. Looking at the map, the only iffy bits I remember were the roundabouts around Harrow-on-the-Hill station and just north of the Northwick Park hospital, neither of which will trouble you. I come from a looong way north-west, and end up a long way east, so I've only rarely done the whole lot as a commute.
@CharlieB used to do a similar journey, I believe.