First, the good news. Even without reading your post a quick glance at the track plan told me "early west country branch with Brunellian overall roof", so you're on target for suggesting the atmosphere you want. I've read through the suggestions and agree with them, and have only 2 to add.
1) The Up home (assuming trains go down to the terminus and up to the junction) might be a bit awkward for sighting, that close to the road bridge abutments (bearing in mind the GW placed their signals assuming right-hand drive locos). 2 possible alternatives are
i) Move it "outside" the bridge, which would aid conspicuity. It could then be made even easier to spot by either painting a white square behind it on the bridge or giving it a "sky arm", a post tall enough to give the arm a clear sky background. The downside be that a train stood there might be out of sight of the signalbox, and unable to observe hand signals for shunting movements (making additional ground-discs necessary)
ii) Leave it where it is and improve its visibility by either giving it a co-acting sky arm visible to approaching trains a good way off, or providing a banner somewhere beyond the bridge (which might not even be on the modelled portion of railway, but we'd know it was there...)
2) The position of your old, out of use signalbox is a bit odd, remote from the station and only just in operating range of its associated pointwork. If I was signalling the layout from a clean sheet I would (depending on period) either put a single box where your platform box is, or have two, one at 6'8" along x 1'"8 in and the other at 17' x 1'. In the days before track circuits, boxes were placed as close to the points they controlled as possible, to a) shorten rodding runs making points easier to operate and less vulnerable to changes in temperature and B) more importantly, give the signalman as good a view as possible of any shunting operations over points he controlled- it's surprisingly easy, in the absence of sophisticated safety equipment, to swing the road under a movement. With a terminus you only have to worry about the points at one end of the layout so my first suggested position would have sufficed for the layout at that stage, but when the extension opens you hit a snag. Originally BoT regulations would not allow a facing point to be further from it's controlling signalbox than 110 yards, that's 1320mm or 4'4" in 4mm scale, and you had to have a good view of every point under your direct control. Some stations managed a long loop from a single box by placing it halfway along the loop, but that would be impractical in this case- that lovely overall roof would spoil the view. The only alternatives are i) control the country end points from a ground frame (usually beneath the GWR's dignity, though they did resort to it on odd occasions, such as Yelverton), or provide a second box at the location suggested, on the opposite side of the line to the first. Companies did that, where practical, to allow both sides of the train to be frequently examined for defects by signalmen as they passed by. Your platform box would date from much later, when improvements in rodding allowed the maximum reach to be increased to 180, then eventually 300 yards (for mechanical points- electric points have no limit), enabling the company to manage with 1 box instead of 2.
SO the upshot of all that waffle is that if you really want both old and new boxes the best place for the old one would be at the country end of the station, on the up side (I would expect that the original town-end box would have been demolished on abolition, since it would get in the way of shunting and yard work). Since this box would have been smaller, having less pointwork to control, this gives you an opportunity to strongly suggest location and age by modelling a small pokey structure from an earlier age, say a GWR type 1 or something stone-built by the Bristol and Exeter.
Hope you find the above useful, or at least mildly interesting.