Hello, I went last year.
We got there at about 12.30 - we were numbers 13 & 14 in the line (I know this as they give you numbers before you went in). It's a long wait, wear some comfy shoes, there is a coffe shop in reception at the BBC, so even though you are outside, one of you can pop there to keep you refreshed.
When you get in, if you want to be sat downstairs (and there aren't many seats downstairs due to production guests) you need to seat yourself near the door at th far end of the room (to the left of the bar as you are looking at it), this is where they call you to when you to in order to go into the studio. The further up the line you are the more likely you are to get downstairs (we were downstars last year). Flash a smile at the people who show you to your seats....if some production guests don't turn up you may get moved to a better seat. I got moved last year, ended up third row from the front, just behing Rav (who was in the audience).
It goes really quickly, so enjoy it all! You can take pictures after filming has finished (but you are not in the studio long afterwards). You will see lots of the dancers/judges etc outside - as you queue up outside the studio when they have walked you over - we saw Len, Darren, Vincent etc.
Bruce was a bit of a pain when we went - but don't let that put you off.
Funny thing - when the VT is on a team of make-up artists rushes on the "touch-up" the judges.
Any questions, just ask