If you time it right, you can get a train direct from Watford Junction to West Brompton, then you just have to walk across the platform for the District Line to South Kensington. However, this train only runs once an hour at xx:52.

Note that the station you want is Watford Junction, not Watford (which is a Metropolitan Line station on the outskirts of town).

Driving to Hammersmith couldn't be simpler - it's right on the A4, just as you come into the city from Heathrow. The road's likely to be busy, but not overly so.

The source of delay, depending on the time of day, is likely to be the ever-tedious roadworks between Milton Keynes and Luton on the M1. If the Hertz station were closer to Milton Keynes station than it is, I'd recommend going from there, if only to avoid this. If you're early enough, though, it shouldn't be an issue.