In London, the "appointment" simply means you're only allowed to rock up at that particular time - but it's not an appointment in any commonly understood sense of the word. There are two slots a day, 9am and 1pm. Everybody arrives and queues from at least an hour before that slot opens. You queue outside, whatever the weather, and of course there are no seats. When they finally open the doors, they control entry verrrry carefully, so the queue shuffles forward slooooowly. You can expect to be standing in a queue for 2-3 hours before you actually get in the door. Then you stand in another queue to hand in your papers, and another queue to pay, and so on. All the staff are about as surly as you'd expect from that environment. But you do get the visa in your hand by the time you leave.

I generally advise anyone going to France to pretend they're going to Germany instead. The German visa office is, as might be expected, a paragon of efficiency. And they're nice!