To London OLD St and Back same day

I have to pick-up a person from Heathrow who is coming to an conference at
OLD ST (Post Code: EC1V 9BW). I have to pick him up with a car as his
luggage will remain in the car. Can anyone advise where is the BEST,
SECURE, CONVENIENT, and CHEAPEST car park to park at and then make our way
to EC1V 9BW either by Tube or SHORT walk. I am travellling to London for
the first time by car and also would like to avoid Congestion Zone.

Can anyone here offer any advice, guidance for a Day trip returning to
Birmingham same day before rush hour.

Yaz