@rookeries Welcome to the forum 🙂
Moving the UK plane to SZ 5 to attack the German transports there indeed is a valid combat move. The battle will be resolved according to the “defenseless transports”-rules on page 19 of the rulebook, sometimes referred to as “autodestruction”.
Scramble is not part of the ruleset here (you might mix that up with the rules for Global 1940).
Also there is nothing in the rules saying that planes must be accompanied by attacking ships.
So never leave your transports unprotected 😄