The last time I tried to get such an excess before boarding the clerk spent a while pressing buttons before telling me to just buy it on the train.
The last time I tried to get an excess was at New Cross Gate. The clerk spent a few seconds pressing buttons and said that the only type of excess fare that exists is Single to Return. However I've never had a Guard deny that an excess exists, and never had a Guard refuse*, or not know how, to issue an excess.
In my experience, there is more chance of a Guard issuing an excess fare correctly than a booking clerk.
(* other than showing discretion; a quite different matter and I don't count that as a refusal).
