Yes, in fact it could even be opened:
http://www.heathrowairport.com/heath...y/hand-baggage
Originally Posted by HAL
Baby food or baby milk: This exception is only permitted if you are travelling with a baby or infant. Please carry only what you need for your journey and, to assist security screening, please do not carry these items in a flask. If you are not travelling with a baby or infant, excess liquids should be carried as hold luggage. You may also wish to take advantage of our pre-order service for collection after security by calling us on 0800 678 5324 or email us.
Not sure how reliable the "pre-order" service is, if it's in the hands of Boots, I doubt that it will work successfully with plain cows milk.