Here is another option, 12 quid travel pass for the southern network (or 24 group pass), however as far as I can see you'd need to buy a return to Portsmouth (10 quid) and then go from there as soton direct to London isn't on the southern network. A lot of p/ssing about, and undoubtedly longer, but an option to save a few quid - 22 quid each or 52 quid for 4 people = 13 quid each on the group save.
http://www.southernrailwaytickets.com/daysave.php
http://www.southernrailway.com/networkmap
Edit: No you wouldn't need to buy an extra return to Pompey as it's part of the network so it's 12 quid each or 6 quid each as part of a group saver.