Your dog needs a trim!~
It is summer here in Australia and so many people have their dogs shaved or trimmed. I decide not to because I have a Cavalier with pretty red and white hair and frankly many of the hair cuts I have seen on Cavaliers here are dreadful. I groom Bella regularly to get rid of the undercoat and she is an inside dog. She isn't matted, and I feel like saying "If I trim her then she will look like YOUR dog!" I love the way Bella looks and I don't want to ruin it by trimming her!
Often it is not a heavy coat which is the problem in hot weather, it's the podgy Cavalier underneath! A dog's coat insulates it in hot weather as well as cold by trapping a layer of air. I had Cavaliers in South Africa and neither of them showed any distress by being unclipped in the summer - I was just careful not to take them out in the hottest part of the day, and kept the house as cool as possible - which is helped by drawing the curtains once the sun starts getting into the room.
Kate, Oliver and Aled