What prevented Jess from asking Miss Edmuds if Leslie could come along with them?

As mentioned in the book Jess likes Miss Edmunds. When Miss Edmunds invited Jess to a trip to the national gallery he realized he hadn't invited Leslie along after reaching halfway away from home. Jess prevented asking Miss Edmunds if Leslie Lelie could come along with them because he enjoyed the secret pleasure of having Miss Edmunds wih him; the two together. :)