The proceedure for dispatching a sliding door or central locking fitted train from a manned station is as follows. Firstly the dispath staff give a hand signal or show a white light, usually suplimented by a whistle, to the guard to tell him to shut or lock the doors. Once the doors are shut or locked, the dispatch the dispatch staff will repeate the signal to the guard to tell him that station duties are compleate. Both of these signals will often be passed along own the platform where the station is curved and the front of the train is not visable from the rear. The guard will then give the right away signal to the driver. With all units, HSTs, and some push pull trains, this is done with 2 rings of a bell or a buzzer (interestingly, 2 bells is the starting signal for mainline trains and buses, but when London Underground had guards they only used one bell). On loco hauled trains the guard will show a green flag or light to the driver, and this might be reinforced with a whistle. This signal also might be passed down the platform by dispatch staff, with a hand signal during the day or a green light at night. In some situations, RA (right away) can be displayed below the signal by a member of station staff after recieving the right away from the guard, and this is the prefered method if provided, since the RA display is interlocked with the signal so it cannot show when the signal is at red, and prevents the risk of "ding ding and away" SPADs where a driver recieves the starting signal and moves whilst the signal is still at danger (both station staff and the guard should check that the signal is clear before giving the right away to the driver, and "OFF" indicators are provided to help them with this, but ultimatly it is the drivers responsibility to check the signal is clear before moving). On slam door trains without door locks, only the right away signal is givern, not close doors, and at unmanned stations it is up to the guard to check the doors are close or locked before giving the starting signal. Where the train is OPO, the close doors and RA signals would be given directly to the driver, and these can also be done through displays under the signal (CD or RA) or at unmanned stations the driver will use a mirror or CCTV to see when it is safe to shut the doors and that the doors are shut before moving off.
Interestingly, driver-guard buzzers arn't always used when proveded. The push pull sets on the WCML were fitted with driver-guard buzzers, as were Cross Country sets when hauled by a 86, but these were never used. Most major stations were fitted with RA indicators, but at other stations a green flag was used. This was alright when driving from a 86 or 87, but when driving from a 90 or DVT, which don't have opening windows next to the driver, the driver had to get up and lean out of the door to see the guards signal. Very odd. Now that the Mk3s and 90s are moving to 'one', they use the buzzers, as they have always done with the 86s an Mk2s.