There is zero chance of it being a 767 then. The last longhaul configured 767 is due to leave BA over the summer. One of these is currently due to go to open skies.
While a 787-8 is scheduled to take over the BWI route there is always the possibility of the occasional 787-9 flying the route. However the first seats will only be used if the flight is sold to sufficient levels and even then will only get a Club World service.
In comparison to the 767 before, a 787-8 has significantly more premium seats so this is very positive for BWI, unless you happen to like the 2-3-2 seating in economy on the 767.