"Standby" and "say again" are extremely useful. Though it's worth remembering that London Information almost always want an ETA, so you'll have it when they ask. However, they are entirely happy with "in approximately 30 minutes", if you haven't worked it out. And if you're crossing the channel, they'll want an ETA at the FIR boundary. Judging by the number of times I hear them say "Let me know when you have it" on a sumy weekend day, not a lot of pilots know that.
Why do they want departure and destination? I don't know exactly...where are our ATC people when we need them? But presumably, it gives them an idea of your route, and maybe who to pass you on to or suggest you talk to. Also, if you have an accident, they'll know who to contact...if you just left your home airfield anyway. And if you didn't, the last airfield will know where you came from before that, etc.