I spoke to a friend of my fathers who was a footplate inspector on the GWR and regularly travelled on the footplate of Kings after having been a fireman and driver on the GWR.
He states that whilst tender first was not forbidden, it was generally only done on empty stock workings out of necessity (such as a possible run from Kingswear to Goodrington on a Summer Saturday when the sidings at Kingswear were full and there was no spare loco available) or where, for some reason, the turntable or triangle were out of order. The main reason being that the driving position on a tender first King was awful. In addition to the driver being "wrong side" of the footplate, the size of the tender blocked the view of any signals and as such relied on the fireman being the signal observer even though the driver was still "in charge and responsible" for the loco and train. It also meant that all movements on the controls were reversed and so it took a lot more concentration to operate effectively and that you had to keep looking round to check all your gauges and devices. A King running tender first was only possible when the timetable allowed for slower running.
In addition, King drivers.. who were the top flight drivers, were not keen on getting their uniforms and persons covered with coal dust that blew down from the tender when running tender first.
He stated that for training purposes it was required to be able to drive a King tender first, but very few drivers would accept that option if any other option were available.
As to piloting a King, he absolutely confirms the statement by the StationMaster in the post above this, but also stated that it took a lot for a King driver to accept the need for a Pilot, even when it became obvious. He said that King drivers were, in his words, "an 'ornery lot" who fully basked in their status as a top flight driver and wanted to be the lead loco wherever possible.