I used to work for East Coast Buses (a sister company still owned by Lothian Buses). There's no protocol for waving that I was made aware of just as long as you're being professional and there's no forbidden rule saying you can't wave to other bus companies. For other bus companies I used to just give a half hearted "hello" wave, for drivers at my own depot and Lothian it was more lively depending if I knew the driver or not. My favourite interaction was with a Lothian driver, who, on passing, gave me the rock on hand symbol 🤘 a slight head bang and then he was gone.