Where in London would be best (least congested) place to pick up a rental car. We're coming into St Pancras train station and driving to Cambridge.
I would suggest you taking the train to Cambridge and picking up a car there. The train leaves from King's Cross which is adjacent to St Pancras. You really don't want to drive in London. You have to pay a congestion fee to drive in London which is around £8, if you don't pay and get fined the fine has to be paid that day or you face paying an extra £120 fee.