I carried baby food on board and it wasn't a problem at all. You can carry the entire pack of baby food if you want, that's what I did because it was easier for me.
As far as change of water, I did not carry bottled water from here. It was too much of a trouble. Just buy bottled water in UK or boil the water (which is not required since tap water is drinking water). The point is my baby did not get used to the difference in water, so, probably yours too will not get used to UK water and when you come back the baby will just drink HK water like before.
Don't worry too much. Like us, babies also learn to get accustomed to these changes.
Enjoy your holiday