Dogfish - to be more certain of getting a same day visa, 4:30-5:30 is the time to aim for. Someone I know, queued from 7:30am last Wednesday and when they got to the window, was given a lemon ticket to return in 10-14 days time.
The situation THEN, was that the HCI was issuing same-day visas up to around the 400-500 mark. After that you're told to return. There have been instances where people have 'cried' emergency - i.e they were travelling in TWO-DAYS time and URGENTLY need a visa; it's touch and go, whether they allow that as a valid reason.
So, they decided to get there for MIDNIGHT - yup, they took sleeping bags, coffee, a good book......long story short - they were number 1 in the queue - by the far window closer to the entrance - and got a visa; out by 09:30am.
Now HCI London opens at 08:30am
Getting there for MIDNIGHT is (at the moment) a little extreme. You may have to adjust what time you get to the HCI, as xmas nears and the season, gathers speed - meaning larger numbers are applying for visas closer to their time of departure.
In
MY case I got there for 5:20 am was 37/38 in the queue, let in at 08:35, at the visa window 08:48, visa collected 09:25. So, the actual process didn't take that long, it's the queuing thats the problem.
No point in my partner also queuing - she was at home sending encouraging texts and chat....which helped.
Have to say, it WAS a mild day.
On the way out (having checked visa: correct spelling, dates etc) I took a pic of the queue.......you don't wanna know
Hope this helps in your judging what time to queue up