A Company spokesman said yesterday:
It's an unattributed source, i.e. someone's been reading pprune and more likely airliners.net
BA haven't as yet indicated any sourcing of additional A318s yet, also given that LCY-JFK is connecting two major financial sectors from the (near) centre of the London financial district, I'd expect to see another LCY-JFK sector before BOS or IAD. I think it's a particularly small and niche market that too many people have extrapolated into being the next big thing.
Westbound loads aren't great due to the stopover in Shannon, even with CBP, Eastbound loads are better.
Now it's not exactly Concorde, but the analogy rather holds up well given the target market. It only ever made money out of JFK with people willing to pay silly money, that's not the case at BOS and IAD.