A two-aircraft base with additional destinations such as 2 x daily DUB and EDI (which I think would work).
You can't put an ATR operation up on high volume low fare routes such as these unless you have a very high percentage of local business (higher than average fare) pax comitted to the service - in which case you basically have a niche market.
It seems to me that Southends main weakness is not being able to take 737-800 ops (correct me if I am wrong) as that will be closing out so many potential operators.