I queued for the 5S at Plymouth and I can tell you there is no point getting there early. I had trouble with the nano sim so didnt leave the Apple store until around 10am. After all the people who had queued had left, the staff went out the back and put more stock on the back table.
In other words, if you have turned up at 9.45am you would have got the 5S you wanted, without queuing. Except no gold like everywhere.
The iPad Air is pointless queuing for. There will be plenty of stock across all Apple stores including Plymouth. Just get there around 7.45am. The queue won't be massive as most people will queue for the new Mini which does have stock restraints. I intend to arrive at 8.30am. Should be optimum time to get one and leave.
And don't forget there will be more staff than customers as well so paying for one won't take long.
BTW what size of memory are you guys getting? I was thinking of 16 or 32gb in white?