I would guess they took the cost of a first class return ticket, where the first sector wasn't used due to them cancelling the flight and credited it against a new one-way ticket home. They were probably similar costs - as you probably know, a one-way is often 75-100% cost of a return, particularly at short notice. The CSO probably thought they were helping - but he has probably screwed up the process here.
The new ticket was issued and used, and then you initiated a chargeback on it. So they feel they are out of pocket for the one-way flight in F, which you used.

I think you are right here, but I would gamble that this is what happened and why they feel you have ripped them off, not the other way around.

A UK261 would have been cleaner.