The RAF and other British Commonwealth Nations used 2 types of hats, the visor hat and the side hat. Officers were allowed to wear a visor cap or a side cap while the other ranks were only allowed to wear the side cap. The RAF didn't use a summer version.

The officers version of the side hat was made of fine officer's grade wool and it featured a 2 pieces badge, the RAF eagle with the Tudor Crown, worn on the left side. The other ranks version was made of blue serge just like the tunics and a bronze cap badge worn on the left side.

The visor hat was made out of fine wool with a bullion or metal/cloth cap badge. Officers who held an Air rank had a single golden bullion design on the peak while the Air Commodors and Marchals had a double bullion design.

The other British Commonwealth Nations followed the same regulations, only the color and badge varied from nation to nation. 

Unlike the US, RAF personnel didn't wear these hats during flight.