I had one three weeks ago. it's not as horrific as you might think (but it ain't lovely either...!)
They will spray the back of your throat (and sometimes under your tongue) with something to numb it (tastes a bit like burnt banana!).
Then they'll give you a shot of sedation and probably (well, they did me) a shot of a painkiller.
Then, as you are beginning to go all woozy (it's nice!), they will put a mouth guard in (looks like one you get at the dentist). This keeps your mouth open during the proceedure. Then they put the camera down your throat. To be honest, although I remember that, I really didn't feel it. They told me I probably wouldn't remember anything but I do remember it all - albeit, it has a kind of dream like quality.
Depending on what you are having done, they might inject a dye inside you (I had this happen). This is the bit that hurt - think of it like an internal scratch BUT it was over sooooo quickly. The staff will hold your hand / expect you to be nervous and, as ever with the NHS, are bloody marvellous.
I would not have wanted it without sedation. If you are having it with, they will keep you on the ward for about 3-5 hours afterwards (you will sleep a fantastic sleep!). They have to keep you in for that time and they won't let you go home alone. If you are in hospital already (I was having it done as another op had gone wrong and was already in hospital), they send you back to your own ward after about an hour.
It'll be ok.