At my daughter's school they start full days (9-2.30) in Reception, or 4 y.o. kindy. I didn't see the point, but she was totally ready and learned a lot.
If you are going back to work, you'll need a helper that you can trust with the children. Firstly, you are realistically going to be working more than school hours and probably won't get all the school holidays off. Your child might also want to do outside school activities like sport or music and you'll need your helper to take him/her.
Also, children get sick. My daughter has been off for about 1.5 weeks out of the past 4 weeks school. Unusual, but it happens. My husband and I have taken time off for doctors appointments and when she was really sick. But there was probably nearly a week when she just needed to rest and recover at home and she was with our helper.