Stock such as the Class 387 allows SDO per number of coaches accommodated on the platform, which should be automatic (the driver just pushes the door release, and the train picks the appropriate number) but can also be done through the emergency override mode, hence the boards showing the number of coaches to be selected by the driver. Indeed, some stock allows routine SDO per number of doors vice coaches (and the override on Bombardier stock also allows it), but this is rarer.
As for the dispatch equipment, there are quite a few DOO dispatch mirrors, and one or two "look-back" locations, where the existing infrastructure has been deemed unsafe at certain times of day or night, and thus there will be dispatch staff will be provided at certain times or for certain longer trains.
One last point: the whistle isn't normally a formal indication of anything at all during the formal steps of the dispatch process, and just used as a generic warning to passengers that the train is about to depart shortly. It's mandatory at one or two TOCs, but not nationally. However (and this is not a dig at anyone in particular) the enthusiast fraternity does often use the term "whistle" incorrectly in place of the indications of "station work complete" and "train safety check complete". CD/RA indicators are activated after these two checks are complete*, whereas without these, a bat, flag or lamp will be used to deliver a white or green handsignal (respectively) to the driver of a DOO(P) train. Whistles may be used as appropriate to warn passengers throughout. Local variations on equipment may apply - but, crucially, it won't just be a whistle!
*CD/RA indications follow the point at which the dispatcher, or Person In Charge of the platform, is satisfied that the two checks have been completed. They are explicit instructions to the driver to take the relevant further action because the checks are completed.
