Ready Brek is a highly processed form of oats which you'll find can cause blood glucose spikes after eating. The steaming and milling process that it goes through to reach that size means that some nutrients are lost which begs the question why eat it in the first place?
Difficult to get but the best form of oats and most nutritios are steel-cut oats, they are simply cut several times and nothing else so take longer to digest, anything slow to digest is advantageous when it comes to bg control. The Rolled oats and Jumbo oats which are more common on supermarket shelves go through the same process as steel-cut but are steamed for a few minutes, still a nutritious breakfast and shouldn't spike in the same way as the highly refined instant varieties.