Actually, according to my quick fare check, flying to SNA via LAX costs less than just LAX. It's like doing a hidden-city ticket without actually hiding!

Here's what I found:
13SEP LHR-BA-LAX
13SEP LAX-UA-SNA
20SEP SNA-UA-LAX
20SEP LAX-BA-LHR
$555.26

13SEP LHR-BA-LAX
20SEP LAX-BA-LHR
$616.26 or $679.26