I've been to an interview for them and they're actually two companies. You have Ocado (which sells groceries) and the real value in Ocado Technology where they make stuff like this.
Their old distribution centers (with some manual interaction) actually run off of Siemens S7 PLCs. These little robots have dedicated controllers probably running some sort of RTOS (ROS if I had to guess).
They have a lot of related machinery designed by them too. The plastic bags are put onto the baskets with a machine designed in house and when I was there for the interview they were trialing a camera system to double check that the correct items were put inside the bags.
Although the tech and what they were doing is really cool, I can't stand working for people that say the following two sentences in a conversation "Strictly no overtime. We don't like people looking at the clock". Although he may have meant something different, the pension terms indicate he didn't. So I don't work for them, although I do the ocasional shop.