Jeremy calls West at 3:36am and has two conversations with West while also being on hold for six minutes in-between. This means Jeremy got off the phone no sooner than 3:44am.
He then must drive at around 84 mph in order to reach his destination 4.2 miles away at 3:47am. This also allows no time for Jeremy to call Julie, get the car keys, and put clothes on.
It's not known how long Jeremy was on hold for. At trial, Pc West thought it was far less than six minutes. Also, Jeremy's journey was about 3.6 miles, not 4.2 miles, as far as I know. Hence he had just enough time to do the trip, assuming he had telephoned Julie and got dressed before he called the police; his car keys were probably in one of his pockets. The trip probably took between 6 and 7 1/2 minutes, depending on how fast he drove (7 1/2 minutes corresponds to 29 mph average speed; 6 minutes corresponds to 36 mph average speed; the police who overtook him mentioned "about 30 mph" or "up to 30 mph" as his speed when overtaken).