I'm trying to chose my seats for a flight in June 2018, but all the seats shown are unavailable?

Is this because it is too far out to book, or have all the seats been booked already?!

Thanks in advance.

(BA2157 in CW)

Side note - It also looks like this flight may be the 4 class 777-200 but not operating first? Is this a glitch on BA website and we are more likely to be on the 3 class version?