Banana is very high on the glycaemic index, which means it contains a lot of natural sugar. Wheatabix and oats are also high carb.
Oat bran porridge may a better alternative because it's high in fibre and far lower in carbohydrates and most people find the taste and texture very similar to regular oat porridge. If you want to add fruit, try adding a few frozen berries, berries are much lower in natural sugar but still provide essential nutrients and adding them frozen to hot oat bran will help cool the oatbran porridge to eating temperature for a baby.
Tesco and Sainsubury's both stock Oat Bran, usually by the porridge oats, and all health food stores have it. You have to cook oatbran porridge but it's easy and quick to make and can be made with water to further cut down on the carbohydrates.
I find that oatbran porridge has a minimal effect on my BG compared to regular oat porridge or wheatabix. An adult serving has 19 grams of carbohydrates and it's a generous serving, a baby sized serving would be less than a quarter of that.