I will be at a work meeting south of Northampton, with a rental car. After staying there Friday night, I would like to go to London for Saturday night and then fly out of LGW Sunday morning.

I don't know much about driving in London... except I really don't want to. I would like to return my rental car somewhere south of Northampton and catch a train into London. It seems the most sensible considering driving to LGW would be all the way around (or through) the city.

My question is, how close can I get to London before I get into the very congested areas, or the area where I have to pay a toll? I was looking at dropping off at Watford, and catching the Watford Overground there. Is that too close-in to London? Alternatively, I could drop-off at Luton airport, but they only have normal train service to St Pancras.

My destination is the South Ken stop, it carries the Circle, District and Piccadilly lines.

Just not sure what is most efficient... I'd rather drop off and get to my destination as easily as possible as I will have luggage. Thanks for the help.