Hi
Not sure if its too late to reply but.... I would suggest Stanmore as its only a mile away from J4 of the M1 and has a large (400+ car park). Basically, off at J4, up and over the motorway, join the Watford Road heading south, third exit at first roundabout, straight over the A5 roundabout and then left into the car park just before the railway bridge. It couldn't be easier.... and not a great deal of London traffic to deal with!
Its quite an easy trip from there into central London on the Jubilee line, or to the City via the Met at Wembley Park.