The above posts seemed to be saying that L Terminals tkts could not generally be used for onward tube travel, but if the purpose of an L Terminal tkt is simply space-related then wouldn't those people then miss out on the onward travel?
Onward Tube travel requires a travelcard, a single/return to Zone Uxx London (to complete 1 single/return journey within the zones specified), or something more cunning.
Say you wanted to go from Clapham Junction to Euston. The ticket would be marked as Clapham Junction to London Terminals in this case, and would be valid into either Victoria, Waterloo, or London Bridge, with additional validity at onward connections to Waterloo East, Charing Cross, Cannon Street, Blackfriars, or City Thameslink as allowed by the Routeing Guide.
However, if you were to purchase a Clapham Junction to South Hampstead ticket, then it would have the cross London marker and you'd be permitted to exit at Euston as desired. It is always worth looking for fares to the first station out of a terminal if you need a different one at the opposite end of zone 1, that way you get the required cross-London transfer included too.