I buy rolled oats and add a few walnuts and half a grated apple and a few sultanas or other dried fruit. I cook that up to make a tasty porridge. I like sultana bran but don't buy it these days. I use weetbix in a couple of recipes but don't like it with milk as a cereal.
I make up my own bircher muesli about monthly, with the basis as quick oats and add linseeds I process myself, spelt puffs, quinoa puffs, some currants, flaked almonds, and some oat bran. I mix and put it all in a large jar. Use 2 tablespoons soaked in 1/2 and 1/2 kefir and almond milk or oat milk. You can really make up anything you want and it will keep for ages in a glass jar. It is yum. When you want to eat, stewed fruit can be added if you want. The sky's the limit really.