We are planning to visit England (mostly Cornwall) by car. At the end, we want to spend a few days in London before driving back to the continent, but we want to avoid taking the car into the city. Are there any good options to park your car for two, three days just outside London and do the rest by public transport?  (Where "good" should take prices, availability, security, reachability into account)

We are planning to visit England (mostly Cornwall) by car.
At the end, we want to spend a few days in London before driving back to the continent, but we want to avoid taking the car into the city. 

Are there any good options to park your car for two, three days just outside London and do the rest by public transport? 
(Where "good" should take prices, availability, security, reachability into account)