The Air Weapon certificate is a doddle to apply for, even when you are shooting in your garden.
The bigger issue you face is arriving here with air rifles WITHOUT a certificate. In such a case, you are holding illegal firearms and liable to find yourself in trouble. It will be a good idea to find a gun shop on the island, capable of storing them for you pending release of the certificate.
I'm down in Argyll and the local police thought it a really big deal at the time.
Oh, welcome to Scotland btw.