Yes, A40 Devons of the early to mid fifties used a 1200cc engine.
I am not sure if it was the "B" series but it was not an "A" series.
Although they do look alot alike.
My 59 A40 Farina came stock with a 948 A series engine and yes a 1275 dropped right it
The later Nash Metros used the B series MGA engine. Again not sure about the early Mets.