Limitations of the south junction layout mean the new platform can't start before the northern abutment of Station Road bridge. For 12 car standage plus adequate standback allowance for signal sighting, the platform will stretch some 280m north thereof, putting the new up starting signal actually out beyond the current scissors connection between platform 1 road and the goods loop (the new platform 0 road). That scissors must be removed therefore to make room and also probably the next trailing turnout from platfrom 1. If a north end connection to Up Redhill from #1 was retained, because of the simplifications required to fit in platform 0, the overlap would no longer be able to swing towards the goods loop with the result that every time a move was signalled in to #1 from the south (from Gatwick, Tonbridge or Dorking) it would likely lock out simultaneous moves in or out of #2 at the north end. Removing the north end connection removes that restriction, resulting in a more flexible layout. The significant reduction in the number of turnouts at the north end should give a reliability benefit as well as a maintenance cost reduction. I believe this job is being accomplished using the existing signalling interlocking. That will have limited designers freedom in what can be changed realistically - i.e the 'blank sheet' approach sometimes possible in a full resignalling would not be available. Generally, removing crossovers and other facilities is fairly easy in an old interlocking but significant remodelling of junctions is not.
With a dedicated reversing bay fewer Reading line trains will need to use #2 surely, which will leave that platform more likely to be free, along with #3 for down services.
A powerful incentive for operators not to make last minute platform changes unless absolutely essential!
That may have been an option looked at, and it could have made sense with the current south junction layout, but there are longer term aspirations to make further changes there that allow additional parallel moves for which placing the reversing platform 'in the middle' makes more sense. Those significant changes are best accomplished when resignalling of the area takes place.
With more platforms available and reversing movements concentrated on #1, fewer short term changes should be neccesary. #2 can be used easily today for down trains. The only problem is overlap conflicts at the south end, although I think there is a warning route available that allows access to #2 simultaneous with a move across it's bow at the south end. That is heavily restricted by approach release from red at the down home though. Signallers have the choice of accepting this, or selecting the corresponding main route, whose overlap locks up the south junction solid preventing all other moves until the down movement has come to a stand in #2.
That seems an extraordinarily negative attitude! The new platform will help to spread the passenger load away from the congested island and subway with most London trains relocated to the suitably generously proportioned new platform with its own new access down to the ticket hall.