Wembley and Shortlands are both in Zone 4, so not touching in and out for the second leg wouldn't actually save very much - only the "TOC premium" (and he would save even more - legally! - by going via Clapham Junction).
A Zone 4-Zone 4 journey made is allowed more time than a Zone 4-Zone 1 journey actually paid for. You could be over time at Elephant, and back within time at Bromley South. (although non stop services such as Victoria - Bromley South, London Bridge - East Croydon or Waterloo- Surbiton are better bets for pulling that trick)