I fear I can only raise some questions, rather than provide answers, but I would recommend looking into the security of various station car parks before making a final choice. I absolutely agree that it would be better parking outside London if you are staying within the Congestion Zone, but I would also be concerned about leaving the car unattended in a station car park for two or three days.

Some station car parks are probably ok, but the commuter ones will empty out every night, leaving the remaining car or two very vulnerable.

As an alternative, you could do a cheap airport parking deal at Gatwick (I always search www.skyparksecure.com) and then take the train into Victoria from there. If you take the normal service, rather than the Gatwick Express, it's much cheaper - both run 3 or 4 times an hour.