There will be milk in Boots with all the problems with liquids. However, each time I've flown, I've had no problems taking through kids cups or bottles filled with juice/milk or water. I've part frozen the bottle, then added more milk or whatever on the morning, so it stays chilled throughout the flight.
We asked Starbucks airside for a little milk for dd's cornflakes and they gave us a great big cupful free of charge. Very charming they were and even offered to warm it too! To be fair we also bought coffee although I'm sure they would sell you some milk if you didn't want to buy anything else which you could then transfer to a cup or whatever. This was Stansted but it'd probably be the same with any of the big coffee chains at Gatwick.