I had a 9am appointmnent, and arrived early at 8.45. Others must have been arriving earlier for maybe 8am /8.30 appointmtes(?). The visa embassy is totally surrounded by metal barircades and the queues for visas forms on the embassy side of Grosvenor Square.
A big fat security bloke orchestrates things here, and tells you to go and talk to a girl with a clipboard nearby. She takes your name, checks your interview invite letter, your payment recipt, asks if you have liquids, cosmetcs, drinks etc (not allowed in) and then points to the queue to stand in. When I arrived, there were probably 15 people in front of me. There were already other people forming a queue for the '9.30' appointments, adjacent to us. If you are carrying stuff, it makes sense to have a bag to put it in. You will have to for anything metal anyway in your bag, or if not they will give you a clear plastic bag to put such things in.
Come your appointment time they'll start taking in your particular queue in one at a time (yellow line style, stay where you are until called etc). Your name is checked and passport checked by the big fat guy. He asks if you have anything metal on you (in which case you need to put it in a bag). This guy is helpful, openly laughs and jokes at peoples passport photos etc., to be honest though I think people are too stressed out to laugh much with him.
Then you file through barricades down the south side of the embassy. This queue seems to be kept pretty short, until you get you get in front of the main entrance, where there is a like a little portacabin. You go through the middle of this, where there is a counter, and they x-ray any bags, check your interview invite and passport and then you go through a detecting arch.
Then, for a visa, you walk all the way around to the back of the embassy, not the main entrance right in front of you, (beyond where you started from) and enter.
You present your interview invite, which is scanned. And you are given a strip of two numbered lables. Mine was #168. By now it was 9.10am. He also checked you had your photo and payment receipt.
You sit down in what is a large room. Reminiscent of an airline departure lounge, with seats for maybe 300-400, with a row of bank branch windows up one side. There is snack-bar area, the courier counter, a photobooth (�3.50 for two pix), PCs to access the automated application forms, toilets.
An automated PA announces 'Now serving customer x, please go to window y'. The windows in this room are numbered from 1-11. The PA goes pretty constantly actually, making it hard to read, study etc, as you are always having to concentrate on it. But after a while you get the hang of what is happening. For some people (in the loos etc?) some numbers get called out up to five times. 'Final and last call for number xyz'
They are calling people to hand over their applications for the first stage, then after a variable wait these people are being called again for the interview. The interviews happen at a second set of windows in an adjacent attached large room. These windows are #12-25. But you are all in one waiting area. So you'll have calls to windows #1-11 in escalating order, but then at the same time to 12-25 in pretty much randon order depending on how long the applications have taken to process. After handing over your application at the first stage, you are not called for interview until they have run through it and are ready to ask you any final points.
Once you have handed over your application, (and been fingerprinted electronically) you sit again and wait. The guy I spoke to though was very helpful, and courtious. If your application is straight forward, you might wait an hour. If it is more complex, like mine, I waited over 4 hours, before being called to interview. They give you a blue piece of paper at 'stage 1', telling you what to expect next, and asking for contact info for the couriers to return your passport to you.
Some people seem to go in and out of the interview area in five minutes, 10-15 seems more normal. Window 17 in the corner seems to be the one that 'problem cases' are called to. It is the only one in a booth with a door, all others are again like walking up to bank windows. The staff at the first stage are 'local', English. At stage two they are American.
In the interview, they sum up some of the main points, when are you going, how long for, who are you visiting. Most of this I had alreay desribed very clearly in my application (I wondered if they had checked it, or were re-checking to see if I could recall what I had claimed). They query anything unusual as well. If they say they will grant you a visa (takes circa a week), you go and hand in your blue courier form back in 'room 1' and you're done.
They might say they need more info and explain what... or other things I can't say what other outcomes might be....
What I will say is that the staff are overall, very helpful, in what is clearly a big stressful situation for everyone. Sitting in a room of up to 300 nervous people for hours was certainly an interesting experience!
The initial application forms are rigorous, (and pretty scary to be honest). It is good if you have done everything correctly. As indicated if you do need to change a passport photo, or re-print the automated application form, there are facilities... but certainly for the PC's (two teminals)... you could be waiting hours. Much better to have got it right when you arrive. (I was not however pleased to have paid �10 to get two US size passport photos from Snappy Snaps, to see the photo-booth in the embassy provides them at �3.50... but then again if that latter machine is out of order, you are probably not in a good situation for the sake of �6.50 potentially saved).
There are some plasma screens up in the first big room.. but they all indicated one line of text 'Disc read error'... so I have no idea, what these screens might usually be for.
By the time I left at circa 2.30pm. They were calling ticket number 400 and something to stage 1, and there were still hundreds of people int he waiting room...
Hope that helps.
p.s. Other little stuff...
You get refingerprinted after stage 1 and right before the interview.
There is a water fountain outside the loo's, so you could take in an empty water water bottle and fill it up here if you like...