Why would Monarch want to operate a flight "just for enthusiasts". The A300s in their current config have 361 seats which is a hell of a lot more difficult to fill on the 180 seat A320s or 221 seat B757s/A321s.
They have done this multiple times. They have even done an A330 with only enthusiasts on.
On a side note G-ZBAB has arrived today at MAN