Titan will be using an A320 on the routes flown on behalf for British Airways. I expect this arrangement will last unless NEO deliveries to LHR allow more A319/A320 to move to Gatwick. Some will move in March to help service the new timetable.
I agree with others that there is more to come given that BA has enough slots for an average 20 more departures per day. At the moment about 8/9 per day have been allocated. I expect their will be some new long haul destinations and some more short haul. As for more domestics who knows! I don't think Newcastle, Belfast and Manchester are viable as connecting flights alone. It would be necessary to encourage regular point to point travellers in sufficient numbers.