This is quite the most incoherent and bonkers thing I've read in a while.
I am a lifelong Londoner. I own 2 cars. Hell will freeze over before I drive to Oxford St.
If i want multiples I'll go to Brent Cross, Westfield or maybe even Kensington High St. Mostly bus or train unless I've got something big to carry.
If i fancy a mooch, there's Muswell Hill, Hampstead, King's Cross, Wimbledon, Camden, Marylebone, Borough Market, Covent Garden... Always by public transport.
Where are these places that you plan for me to "park for cheap" and then get a bus?? Anywhere outside the Circle Line is too far away. Anywhere inside the Circle Line is full. Really, really full. Buildings above the ground, intersecting tube lines below it.
Bonkers.