Opening bottles / cartons does not make them unfit to use
Even heathrosws own rules don't have you opening evcery bottle. You should get a supervisor down if they do insist and have a copy of the rules with you. Get a looning grip FFS
From heathrow FAQs
Can I take milk powder with me to make up a bottle of milk on the plane?
Yes, you can carry milk powder.
What about bottles or cartons of prepared milk or sterilised water? Do they need to be a maximum of 100ml?
If you are travelling with an infant, these items can exceed 100ml and do not have to fit into the resealable bag. However, the amount must be sufficient for your trip only and you will be asked to taste 50 per cent of the containers. There are no limits for formula milk powder, and bottled water is available in shops after security. Your airline can provide water during the flight.
Can I buy cartons of formula milk from the shops after security?
Yes. Boots sell ready-to-drink formula in both 250ml and 500ml sizes after security. Every country which is implementing the new security arrangements has accepted that parents with babies should be able to carry sufficient baby food/milk for the journey, so "reasonable amounts" are exempted – although you will be asked to open and taste at least 50 per cent of the containers when going through BAA's UK airports
If you wish to reserve formula milk for purchase in the shops after security control, please email genres_collect@baa.com.
What happens if the baby can only tolerate non-dairy and needs soya milk or rice milk?
We are required to test at least 50 per cent of the containers that you carry with you. The milk should be in bottles.