I read on another thread that staff representatives (unions, etc) had some insight into the cab layout for that fleet, and that this is fairly common practice, to try to avoid situations where trains are delivered, and then drivers are reluctant to use them because they're too cramped, have poor visibility (especially relating to signal sighting), or other issues (eg those caused by the erstwhile curved windows on the front of the 385s).
Usually a compromise will be reached on comfort, although this can be a case of who blinks first! Perhaps in this instance SWR's assumption of what a reasonable compromise would look like are way off the mark, as far as ASLEF, etc, are concerned - hence the impasse.
Either way it'll probably be expensive, in terms of redesigning, building, testing revised cab layout and/or new trains sat around unused, whilst the trains they are to replace are promised elsewhere (eg 707s) or incur additional costs to keep them running beyond what was their expected withdrawal dates.
This is likely to endear the railways even less to the Treasury - everyone could end up losing, as they did in other industries.