Most airlines that operate C-Class only BBJ1s or A319CJs have 48 seats installed.
The cabin of the A318 is just 2.5m shorter than the cabin of the A319, yet BA only plans to have 32 seats.
Is that because of range issues no more pax can be accommodated? Even with 38 or 40 seats the A318 should be reasonably comfortably, particularly as the main selling point of the service will be the gateway LCY and not the cabin of the aircraft.