Yes, the Havengore. Ran by the PLA for many years and indeed did take Churchill down the Thames, my father was aboard that day and was presented a commemorative medal for his services. He wasn't the coxswain - second I think.
Used to berth alongside up at Tower Bridge before moving down to Gravesend onto the PLA moorings where the Maplin also PLA ran hydro graphic survey out to the far reaches of the Thames.