Usually if my milk is chilled, I'll continue to put an ice pack with the milk when I go out until she is ready to drink the milk. In your case since you're going for a dinner function, it will be quite easy for you to warm up the milk. Just ask for a bowl and some hot water from the waiter to put your bottle in.
As for pumping and bringing out freshly expressed milk, I usually keep the milk in an insulated bag and feed within 4 hours :)
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