Am I right in thinking he was the last player to score from open play for England?
Nah.
Free kicks, corner kicks, goal kicks etc are taken after the ball has become 'dead' so I would go with the 'not from open play' school of thought.
My query here, without worrying too much about it, is when does a set-piece routine become open play? I think if the ball is cleared to the edge of the box and then rifled into the top corner that is a goal from open play, but is that universally accepted? What if a defender gets to it but only flicks the ball into the path of an attacker in the 6 yard box? How about a free kick that hits the wall, if it falls to a player in the box is it still a set piece goal, if it comes out and gets crossed to the same player in the box to score is that open play?
Tyrone Mings should be in that England squad. Maguire is fucking useless at the moment and it's no surprise that his club are performing way better without him.