It's expensive to have a car in central London because of the cost of parking. It has basically been unbundled from housing; garages sell for upwards of £100k, guaranteed parking spaces somewhat less, on-street parking is either banned, charged for, or very heavily competed over.
As a resident you get a 90% discount on the congestion charge, so driving it will cost you £1.50 per day. I'm not quite sure whether that applies to parked cars as well.
Car parking for residents is heavily subsidised by London councils, a policy which encourages car ownership. Residential parking permits are very cheap (often as little as £100 or so for a YEAR of parking), where as outsiders coming in to park will be charged a small fortune. [1]
This system discourages schemes that could reduce car ownership, like car sharing, car rentals, etc, because residential parking permits must be linked to a particular plate number.
[1] It's true that some newer housing developments have been approved on a "no parking" basis, which means if you live in that building you can't apply for a parking permit. But these developments are still the exception, not the rule.
As a resident you get a 90% discount on the congestion charge, so driving it will cost you £1.50 per day. I'm not quite sure whether that applies to parked cars as well.