One of the answers you wanted is in your own post. BA ordered 12 A318s with options on another 12.
What's the future of the A318? It's like the B736. Neither of them is very economical. It might make some sense for airlines with a large A319/320/321 fleet who need a plane with 100 seats. Otherwise, airlines are better off looking at the ERJ-190, CRJ900, 928Jet, that is if Fairchild Dornier survives, or even the B717.
Why did BA order the A321? IMO, they made a wrong decision of ordering the A318 to replace some of BA's B757s which BA sold to DHL via Boeing when they ordered the A318. With the A318 delay, BA has the opportunity to correct that mistake. BA doesn't really need the B757 range. The A321 is probably a better fit for them. If the A321 works well in BA's fleet, they can always order more as they have quite a few options on the Airbus narrowbody.