You are incorrect in your prediction. It will require round about 15 pax per flight to be in profit. This also does not take into account any follow on business we may get from Americans using this service for the first time and opting to use BA in the future.
The exercise is very different from EOS etc, it has the backing of BA and an investment that will permit periodic losses. Futhermore at JFK / LCY BA are already established therefore there have not been any significant start up costs in ground handling / though there is a direct engineering cost at LCY. This may well change if the management of the A318's and BACX are combined into one operation from the passenger group / engineering prospective, which can only be a matter of time.