As you will see here:
http://www.brfares.com/#expert?orig=WAT&dest=VIC
there is a Victoria to Waterloo, Route: Not Underground, fare for £3.20 that seems to do the trick. There's even a PAYG Oyster fare quoted and the booking engines will sell the ticket. I did wonder if a return from London Terminals to Clapham Junction might be best but such a thing doesn't seem to exist any more, other than as an Anytime Day return, which is a bit steep at £5.90.