Northwest London adjoins the county of Hertfordshire, so it would depend on which part of northwest London and which part of Hertfordshire.
Hertfordshire
About 45 minutes by Tube. If there are no signal failures on your line.
From the mouth of the Thames at the North Sea to Teddington Lock in West London.
Worthing is about 65 miles away from Centrel London
Cheshunt is located 14 miles (23 km) from Charing Cross, making it one of the closest parts of Hertfordshire to Central London.
According to AA route planner it is 21 miles and will take about 58 minutes to drive.
Depends where in Hertfordshire you're starting from.
Take A127 LONDON (to M25), from Basildon, to M25 (WEST) towards STANSTED (to M1).Take M25 (London Orbital), around London, to M1 to The NORTH to LUTON at J21.Take M1 up to A1 to The NORTH towards WETHERBY.Take A1 to CATTERICK.
About 500 miles
It is an estimated 63 miles between Andruel West Sussex and Central London.
The World is not that large. You cannot go that far North!
About 70 miles