I too have been wondering what the score was with the GNER blue liveried mark 2s in the latest Harry Poter film: They're actually seated carriages as opposed to barrier vehicles as
hstmatt says, but the private owner number series numbers that they carry (In the 997XX series IIRC, I think one was 99721) don't tally up with any mark 2 air-con vehicles.
The film used this rake on one side of the platform and the Pretendolino on the other side in order to mask the filming from the rest of the working station and as
Yew says to keep other train movements out of shot, and I think it was done in an attempt to retain continuity between the early Harry Potter films and this newest one, which is a major criticism that has been levelled at many of the earlier HP films, as in the early films then-GNER operated mark 4 rakes were indeed used during filming at Kings Cross. Ironically though, in their quest for continuity the film makers have actually failed to accurately portray the nature of Britains' constantly changing railways, where GNER are no longer the operator of the East Coast in the unceasing TOC merry go round, and will be but a distant memory twenty years from now when that particular part of the film is set!
Clearly East Coast refused to provide any stock to sit in Kings Cross doing nothing, or were asking for too high a price, to cause this situation in the first place, as the unprototypical situation of GNER and Virgin liveried stock sitting at the same London terminus has also been created!
