Yaz wrote:
Is it not better to head towards a Northern Line served station and
park there?

Not really, since that would entail a slow and congested rush-hour journey
to somewhere in the south or north London suburbs from your starting point
to the west. The minor convenience of a direct Tube journey on one train
would be well outweighed by the inconvenience of the car journey that
preceded it.

By the way, I think you misunderstood my query about your subsequent
journey. You said that the other person's luggage would remain in the car,
so presumably you'll be going back to the car with him later and then on to
somewhere else. If that somewhere else was near the Northern Line, for
example, it might make a difference to the recommendations.
--
Richard J.
(to email me, swap 'uk' and 'yon' in address)