The route down (and up) the A1 is far more pleasant than the M1/M18 route and at least in terms o fHarrogate/London is roughly the same timescale. I avoid the M1 because of the roadworks by Luton.
The M1 drops you off at Staples Corner with a short journey east of a few hundred yards before you turn off at Brent Cross to take a right turn over the A406 onto the A41 southbound and onto the Finchley Road about a mile later - be in the centre or outside lane as you go up a short hill as you will turn right at the top at the lights. (You will see an Esso garage on your right.** Saturday traffic should see you into St. Johns Wood near Lords Cricket Ground in no more than 30 minutes.
The A1 joins the A41 at Apex Corner in Mill Hill. Follow that southbound until you go to Fiveways Corner where the A1 goes to The City and the A41 goes to the West End. There is a BP garage on the right about 400 yards north of this junction. Make sure to be in the centre or outside lane. Take the West End route and you are just 2 miles north of Brent Cross on the same road you would join from the M1/A406 route.
I wouldn't use West End Lane as it is a narrow winding road that can have delays - just stick to the Finchley Road (unless of course her address is off West End Lane :-)) )
** When you turn right onto Finchley Road, be in the outside lane if you want to use West End Lane as you turn right into it at the next set of traffic lights about 100 yards away.