I've been in a similar situation twice now, first time was on behalf of a colleague who missed an EU>LHR return leg, so I called BA to book a single back. Very expensive so I asked for the cheapest return possible, to which the agent didn't appear too pleased but allowed me to book it even though he know my colleague had no intention of using the final leg.

Second time was recently when I was in Europe and needed to come back early (again on a cheapo ticket) - I called BA and asked for a single back. Again very expensive so I asked for the cheapest return, the lady on the phone said "Are you aware of the fare rules, we cannot sell a ticket if you have no intention of using the last leg, I suggest you book online" (or words to that effect).

IMO I think there would be a public outcry if BA decided to charge someone because of this. Whether or not the T&C's state this I would be sure, in the real world this would NEVER happen.