I suppose it all depends on what you are planning to do in London. Are you intending to see the main sights? Or do you have ideas for things that are out of town and off the typical tourist trail?
A car is not the easy or inexpensive option if you plan to stay in London. It's rather different driving over here compared with the US, with the non-grid road system, unfamiliar signposting, driving a stick shift on the left and the heavy traffic combined with difficult/expensive parking.