OK there isn't much on this in forums anywhere about the I-130 petition for alien relative and I just had my interview so hope the below helps.
I am a UK citizen. I married my American wife in the UK a year and a half ago and we are now in the final stage of the immigration process. The thing that took the longest was my Police check for Australia where I spent a year. They want payment in AUS dollars from HSBC as they don't recognise some other banks.... I'd suggest doing this and the UK police check early on. Everything else takes time and happens in order for a reason.
The US embassy in London is on Grosvenor square and is huge. The queue outside is long and unsheltered and yet they will still confiscate umbrellas. Also the notice you get from immigration tells you you cannot bring in phones or Backpacks/luggage (a daypack is fine). What they should also say is that you are not allowed: Calculators, memory sticks, mp3 players, earphones, car keys that have an electronic fob for alarms/locks...- basically anything that can transmit or carry and electronic signal. Store them in 'left luggage' at one of the stations BEFORE or give them to an accompanying family member that waits for you outside. Do not try and stash your phone in the hedge outside like one girl. A nice man with a machine gun will ask you to move it.Have your passport and letter for the receptionist ready.
Once you get through security you head to the visa reception and give them your letter detailing who you are and what you are there for. You'll get an appropriate number and then head into a huge room with the digital displays so you can see where you are in the queue.
You will be called twice over all.
After 25 mins of sitting (immigrant visas seem to take less time than fiance/business/student temp visas etc) I got called. you go and stand at a window like at the post office only with privacy partitions. The person on the other side goes through your forms, checking and signing them off. You submit your photos (if you don't have any there is a booth available in the big waiting room. Also the british standard size seems fine for the US paperwork). After 6-7 minutes, All being well , They'll tell you to go sit down and wait for the second call.
The second call, about 30 minutes later in my case, was the interview. Same type of window. the teller will ask about anything that stands out on your application. I had married the same girl 10 years before (long story) and was asked a few questions about this. Really it seemed like a formality, like they were going through the motions and all the stress and lost sleep over what trickey and obscure date and time and history questions they might ask was for nothing. It is a process of ticking boxes that meet their criteria. If your application is in order it should take no more than 6-7 minutes again. I still have paperwork to supply from my sponsor in the USA (no amount of letters, e-mails and phone calls over 6 weeks could get them to tell me what i needed until the day - more stress) and my application is suspended but I get the feeling once I courier it to them (they give you the couriers details) they will process it within a couple of weeks and courier my visa pack back to me. It is not a stamp in your passport by the way but a sealed pack you give to immigration upon arrival in the US. Unsurprisingly there are dire consequesnces if you open that sealed pack.
I arrived at 8:30 am and was out again by 10:15am. It's hard not to be stressed about it but bear in mind it is really just a process. As long as you have submitted everything they have asked for (particularly fingerprint based police checks for countries you have lived in 12 months or more) then you should be moved through the system without much fuss. Any one form can be a stumbling block so be thorough and cover all your bases.
I think temporary visas where you may be a risk of staying in the US after the designated period require more grilling and proof of ties to the UK. Getting married is more about making sure you are sponsored and wont be a burden.
TRAVEL: Another thing is if you are worried about getting to London and getting around you can drive down the M1 and park at Stanmore Tube station in north London. It has a huge car park (£3 all day), is right on the motorway, and is the start of the Jubilee line. The Jubilee line goes straight to Bond Street where you get off and walk south from oxford street to the embassy on Grosvenor square. 35-40 mionute tube ride. 10 minute walk. All day travel pass £11 for tubes and busses.
I hope this helps to relieve some of the stress and suffering that is caused by this long and expensive process. It is just a process but without a lawyer can be confusing and difficult as like anything, if you don't have the experience you find out what could have saved you time and money only [i]after[i] the event.
If you have any queastions i'll try to help.