Stanmore is just off the M1 junction 4 and is the last stop on the Jubilee line. Can't guarentee the safety of parking but it has been OK in my experience.
Hi, I have done this trip many times with no probs. High Barnet is just a few minutes from the A1. You can usually park in one of the streets near to the tube station i.e. Milton Ave/Bedford Ave. I have never had to walk more than 5 mins to station.