Baby milk on flights!!!!!?????

Hi! Flying to Gran Canaria a week on Tuesday, I have pre ordered 8 bottles of baby milk from the airside terminal of Manchester airport for the outbound flight. Just wondering can i order milk from the airport in Gran Canaria? Do I use powdered milk and ask the air hostesses to help me make up bottles? Can I take my previously purchased bottles of milk through their security or will it be opened and therefore unsterile???? Please help and advise. My baby is 4 months old and feeds (every 3 hours ish) on demand.