First Class appears to be designed to be where it is, as little sense as that makes. When the 387s were used by Thameslink, only a very small section was First Class, between the cab and the first set of doors. The dividing door was completely pointless as it was in the middle of Standard Class.
Looking at the seating layout, it seems likely First Class is now in the location it was designed to be, as with the exception of the priority seats, all seats are at tables. If First Class was the other side of the door then there would be too much First Class provided for the majority of routes, and the seating layout would be the same as Standard Class, which I guess would go well with the identical seats.