I believe BA is one of 3 or 4 launch customers for the 318, Air France certainly being one of the others, and there is definitely another customer with the PW order, though I cannot recall who at the moment. Heard the rumours myself a few months ago, when the PW engine was going through some real development problems, which will result in it being late anyway.
Cannot see the point in only operating 6 318s: engineering training costs cannot justify it IMHO unless BA can do work on those of other operators (of which there are worryingly few), and not only was the 318 was not only destined for the regions anyway (which BA have decreased involvement in), but are also heavy aircraft compared to 717 for example.
Agree that 321 should always have been the choice, especially since the 757s need replacing anyway due to age - silly decision in the first place.