"Since you question BBC employees because their pay comes from the public purse, what about all civil servants, local government employees etc."
Yes; above a certain agreed level yes. Same argument. Have folk never been amazed when something gets into the press about what your council has paid someone ? Umpteen times what you get for doing you responsible job well. Some salaries/fees are so large one is almost obliged to think must be nepotism between friends in the favoured group, but that can't be right, can it ? Perish the thought.
As for threats of walking, if someone thinks they are worth more than they actually are, then let them walk. That'd bring salaries down eventually, and the companies they walk from will still thrive but with lower costs.
As for what is too much. Folk will have different opinions, and differentials are not unreasonable so long as they are not excessive, and are justifiable. Surely any debate will be able to come up with a compromise on the maximum between thee highest paid by the company and the lowest ?