As I understand it you can use cow's milk when cooking your baby's food and that includes making porridge from around 9 months. They do however warn that if there is a history of allergies particularly to milk then you should avoid introducing cow's milk in any form until after they are 1.
I have pasted an excerpt from an English website about introducing cow's milk below.
"When can I introduce cow's milk?
You should not give your baby whole cow's milk as a main milk drink until after the age of one year. From 6-12 months, during weaning onto solid foods, small quantities of whole cow's milk can be used to mix your baby's foods.
Semi-skimmed cow's milk should not be given as a drink before the age of 2 years, but you can gradually start to give it after this age, provided your child eats well and is growing well. Skimmed milk should not be given as a drink before 5 years of age."
HTH but if you any concerns try to speak to your health visitor or doctor first.
