At international level I think if you have a decent set of players, and a tactical plan you can fit around them, you are halfway there. The tricky part is getting all the players on board and managing the pressure/expectation. Those bits Southgate seemed to do fine on but then probably because he was more focused on pandering to the media and his favoured players than he was the tactical plan part.
You could see from Spain's first game how hey had been set up to play. They have an abundance of players who are good on the ball yet they set up with two very direct wingers and an out and out striker because their manager obviously thought they needed a goal threat to go with the ball retention. They then consistently picked the same team, injuries and suspensions aside, and played the same way.
England went into the tournament not knowing what their midfield was and not knowing what positions two of their front 4 were even really playing in. They then completely changed their formation mid tournament. One of the two of our front four who actually WAS in a position they knew how to play, ended up at wing back. If you asked me to explain what England's tactics were. Other than "hope the other team don't score", I wouldn't be able to, and I bet none of the players could either.