Eeh, it's all a bit six of one and half a dozen of the other! My tests have all been (haha, the fact I've had more than one doesn't really imbue you with confidence in my advice here, does it?!) in the afternoon, between 1 and 2.30 I do believe.
Early morning: Rush hour. Disadvantages - uber busy, difficult to get out at junctions, navigate roundabouts, more likelihood of people waiting behind you when doing your manoeuvres, you've got a lot more to deal with. Advantages - traffic jams = more time sitting doing nothing, less time driving, less time to make mistakes!
12-1: Lunch hour. Possibly people nipping out to get their lunch, I don't know really.
3 onwards: Schools come out. Kids crossing roads without looking, parents parking all over the place, general busy-ness.
If you can get a Saturday morning test, I think that'd be best. I notice when I have driving lessons early on Saturday morning, about 8-10am, the roads are nice and clear and it's all a lot more relaxed. Failing that, perhaps about 10-11am, or 1-2pm.
Ooh, also, consider when you're at your best. If you're not a morning person, will you end up being in a rush if your test is before lunch? Or will you get too nervous and psyche yourself up too much if your test is in the afternoon and you've got all morning to think about it?