Do we actually know that the reason for disruptions to the 223 is that it may be a target for terrorists? Why only that flight?
There are loads and loads of other flights to IAD and indeed to the USA by other carriers as well.
It would seem that Terrorists are generally not stupid and I am sure that god forbid if they wanted to try something, considering the high focus and profile of the 223 in recent weeks they would just try another flight. It would be foolish to keep harboring on the BA223.
It was suggested in similar threads a while back that there were other "behind the scences" reasons of paperwork, procedures and compliance that might have been the issue, not to mention the sky marshalls can of worms.