If you are a British vessel then the Red (or blue or white if qualified) Ensign is the correct ensign. The courtesy flags are just a nice touch, as Snowleopard says, and hoisting one reminds you to take down the French one.
PS Is that a new Avatar Snowleopard? He is a fine looking feline!
Unlucky- been to Guernsey a few times now and never seen the customs guys- just post the declaration in the box at the end of the pontoon and tke the yellow flag down.