There are a few misconceptions here.
Kelloggs do not manufacture own-brand cereal for supermarkets in the UK. They do make cornflakes and a few other cereals for Aldi in Germany due to a pricing dispute in 2000 with a previous manufacturer, but the cornflakes are not on sale in the UK.
The majority of UK supermarket own-brand cereals are made by Pioneer Foods in Wellingborough and Peterborough. You should be able to tell a Kelloggs cornflake from any other by just the shape alone due to the patented machinery they use.
Don't be too taken in by the nutritional information you see on the pack when comparing Kelloggs to own-brand cereals. A great deal of leeway is given thanks to EU regulations (that we still stick to) over these declarations. They are "typical values" and can vary wildly. So wild in fact, that it's permissible for the same cereal to have different nutritional data on branded and own-brand products without contravening food regulations. The "typical value" figures have a wide interpretation in food regulations due to complicated issues. As an example corn can have a hugely different sugar content based on where and when it's grown and the variety can play a part. Even salt data is flexible.
Nowadays, it tends to be the supermarkets that are eager to reduce salt, sugar and fat in their own brand products and their food technologists tell the manufacturer what they want in the recipe. Sometimes, for technical reasons, it's not possible to reformulate much though.
At the end of the day, if you held a box of supermarket cereal in one hand and a big manufacturers equivalent product in the other, and noticed exactly the same nutritional information, what would you buy next time? There's a lot at stake here.