I think packeted breakfast cereals are a minefield. The flashes on the packet may promote a healthy food but for example the 'whole grains in Weetos amounts to only 44% of the ingrediants, they also contain 'Wheat Flour,(ie non whole grain flour) Sugar, Maltodextrin, Cocoa Powder, Vegetable Oil, Whole Milk Powder, Salt, Potassium Chloride, Flavouring ' + a quota of added vitamins.
The GIs of packet breakfast cereals are with odd exceptions like frosties ( :!: ) usually high, probably because those 'whole grains' have been squashed, chopped into tiny bits and then added to high GI ingrediants , before being extruded into interesting shapes.(the extrusion might actually help lower the GI a bit... thats what happens with pasta' )
When you add milk to most cereals they become very soft and very easy to digest quickly.
(think what happens with potato.. when you cook it and mash it ,it's GI rises)
Oats for porridge have a lot less processing but are relatively heavy so portion control is important (30g per portion on a French packet..at least 10g more in the UK, wonder why?) adding some berries can make a bigger helping and add one of your 'five a day' to your breakfast.
and be careful of 'instant' porridges as again they are more processed.
Another alternative is making porridges or muesli with a mixture of flaked grains and seeds but again portion control is crucial.... and it's expensive at the start as you have to buy lots of packets from health food shops.
Your meter will guide you as to what you can eat from this minefield, and how much of it.