Hi David,
Sadly this is one of the problems of the different systems. As long as once you’re in zone 1 you stay in there then there is a way to avoid paying extra. Travel from Clapham Junction (or stay on the train from Kew Bridge) to Vauxhall and exit with your Key card there. Then go to the Underground and use the Oyster card. Vauxhall is dual zoned so you’ll be fine with both cards.
Alternatively, if you might cap at the zone 1-2 rate (just over 3 zone 1 rail journeys) then there’s no penalty for switching at Clapham Junction in zone 2 because there is no zone 1 only cap.