I doubt LGW would want a D328 taking up valuable peak slots even if the fees were paid. The 20-odd pax on the Dornier would have to spend a lot in the terminal to match the income from an Airbus.
Aside from that, STN is actually better placed overall for the City and Canary Wharf. Technically Canary Wharf is quicker by rail from LGW but the time saving is offset by the longer flight. STN is also pretty useful for Cambridge, I believe there is some academic/research traffic between the two.