That team could easily be 4-3-3 as well so there's a bit of flexibility with it. Mount could drop into midfield with Rice and Bellingham easily.
England will be expecting lots of the ball so want options high up the pitch. Having Mount at 10 means England can play through the lines a bit better. Mount can take the ball on the half-turn easily and get England moving quickly.
Defensively Declan Rice is key, as he'll need to be on song to stop Iran breaking quickly on transition. I expect Iran to sit deep, get England high up the pitch, then attempt to win the ball back and break quickly against a disorganised defence.