Depends on the area/ child/playgroup etc.
I moved from an area (part of london) where "nursery" was attached to a school so I fully expected just to send my daughter when she was 3 1/2 to nursery in the september the year before she went to school. I hadn't at all planned to put her anywhere for a few months before, when funding started as I didn't want to chop and change.
I moved to New Area (town out of london) when daughter was just turning 2 and everyone kept asking if we'd signed up for a pre-school yet. At that stage I wasn't at all ready to let go of my daughter and wasn't sure how she would take it but I did go and visit all the local ones and found a fab traditional one run by older ladies, just upstairs from the mums and todds I was going to and put my daughters name down.
She will start end September at 2 years 9 months - the youngest that group takes them and will have 2 years before school (there are no nursery years here - schools start with reception). She is so different to when she just turned 2. Talks non-stop, desperate to play with other children, loves any new stimulation, will go to a couple of trusted friends for a few hours no problems. Both daughter and I are quite excited about her starting!
A lot changes in their 3rd year. My daughter would have had a hard time starting at 2 (but I'm sure would have settled after a few weeks) but at 2.9 she will have a whale of a time. She is quite confident socially, bright, curious about the world etc and will fit in fine. If I had a slower developer next time I wouldn't nec send him/her before the funding kicks in.
The 2 mornings thing is because that is far less disruptive for the child. 2 lots of 2-3 hours really won't harm a child and for most children be a very positive experience - new activities, crafts, input etc.
Not that anyone "should" do it if they don't want to!!