I keep it in an ice cooler bag while travelling and then put it back in the fridge if it wasn't touched. Of course if the temperature while in the box stays cold.
I did it multiple times. Make sure your ice packs are really cold and that your bag stays in a cool place.
If you have a cooler bag with a freezer block or something get a little thermometer! As long as it stays within fridge range 0-8C you are fine :) I’ve done this loads of times x
The 24 hour period is for breast milk that has been thawed from frozen. If it’s been frozen, then it’s good for 24 hours once fully thawed. If it’s frozen but partially thawed, as long as it still has ice crystals it can be put back into the freezer.