If you rent a car, you'll have to pay a daily tax imposed on all car traffic depending on the locations in the city you drive. I think the areas around parliment are most expensive since it's the most "touristy". And you will have to pay the tax. There's security cameras on every building and intersection in London. They WILL find you.
We rented a car on the last day of our trip in London when we were headed down past Bath and into Penzance. If you've never driven in England, it's an experience. The roads are like glass. Incredibly smooth.
If you are just staying in London, don't get a car. Parking is rediculous and you can get anywhere by foot using the Underground. Just remember to "Mind The Gap".
