I suspect that however early you turn up the queue will already be streaming down the street (of course, it will only get worse as the day wears on). The truth is, of course, that there are bound to be many people there who are not professionals in any sense of the word and who (forgive me if I place my cynical hat on my head here) will probably have been prepared to pitch a tent outside the studio the previous night in order to be in with a chance of becoming a star. So, I would suggest getting there in good time (as always) and don't be surprised that it still takes forever for the crowd to shuffle on through and to even get given your number (or whatever they do). Still, I hope you *will* be seen if you get there in good time for the audition.
The Evian bottle tip sounds very clever - obviously, all people in TV land fear the reprisal of the advertising companies, so anything carrying a dreaded 'logo' cannot be shown on prime time television for fear of (shock, horror) LOSS OF CASH!!! On the other hand, if you want to be in with the chance of appearing on TV during the audition (perhaps you don't), I'd try and avoid wearing anything with a logo on it as well (they won't film you in a Reebok top...oh, no).
Clearly, I have not got the faintest clue about what Machiavellian plotting goes on behind the scenes as decisions are made as to who gets 'put through to the next round', but by and large, it splits into the 'talents' and the 'idiots':
I trust everyone who has been discussing Nancy is a true talent, and to 'prove' that, I guess they will be thinking about your look, the actual quality of your voice, the strength of your acting perhaps. You will need to look as professional as you can, perhaps with a hint of 'Nancyness' about you. I don't know quite what I mean by that, but simply, to show them that, yes, you could be capable of playing that archetypal 'tart with a heart'. I wouldn't advocate dressing up in a Victorian Madame's costume, however, unless you wish to be labelled an idiot. But dress with a bit of style, and make the most of whatever flatters your face, because they will be looking to cast someone who they think the viewers at home can root for/aspire to be. If you are young (and I think Nancy is typically cast in the late 20's - mid 30's range), then make sure you make the most of the fact if you get asked about it - it might be a selling point, depicting Nancy as a teenager. Be strong in yourself, and try as hard as possible not to let nerves cut in (although like any auditioners, if you are good, and they see you're nervous, they will let you off). They will never turn down someone they feel has genuinely offered them something remarkable. Whether you can provide the sort of performance that would light up a West End stage (or even a TV set for eight weeks) is your call.
If you wish to be seen as an 'Idiot', then I think the criteria are a lot more straightforward; you dress in a stupid costume that will make you stand out from the crowd, you come up with the most awful performance routine it's possible to think of, you adopt a ridiculous gimmick (coming with a violin, depicting Nancy 'after she's dead', bringing a dog to play Bullseye, being a man in a dress etc.), and you take every opportunity to talk to the camera people whenever they look with interest in your direction. Of course, I'm not thinking that anyone who has discussed these details will want to look like anything other than a total winner, but you never know, someone else may be reading this. The serious point is, I suppose, that always these guys are going to be looking for what makes good TV as well as what will be good for the MT scene - so, anyone who comes dressed as a cowgirl and is tone -deaf will probably be in with the chance of two minutes of airtime. There will be people there on the day who have no other intention than getting seen. Still, to become more likely to be picked as a 'talent', I think you will also have to have something that makes you stand out from the thousands around - an interesting look, a particular style and so on.
I'd say, if you're going, and you hope to be in with a serious chance of getting any further with it (as opposed to just going for the experience, which is fine), you need to give some thought to that - what will you sell them? What will they see of you when you walk into the audition room? In that sense, it's the same as any other audition.