I reckon England and a lot of other teams will work out their best XI over the course of the tournament. The group stage can be an opportunity to chop and change the line up. I feel like going into the tournament most of the favourites don't have a defined starting XI, bar Italy. This mainly down to injuries and the lack of preparation teams have had in the run up to the Euro's (normally everyone would have at least a month to prepare, this year it's been 2-3 weeks max).
England are probably the most effected by the tight schedule this year. I believe 12 of the 26 players were involved in both the EL and UCL finals, so by the time they arrived at the England camp, it was going to be very difficult to give game time to the 12 that arrived late. Rashford and Shaw played 75mins which is good to see, but I can imagine Southgate just wanted the others to have a full weeks worth of rest before chucking them into a game. I know Werner got a run out for Germany, but honestly is there a massive benefit giving them playing time? In my opinion, I don't think so. Just look at the way Austria and Romania were kicking lumps out of the England players, the risk of injury outweighs a 45min run out. Hopefully through the group stage Southgate can figure the side he wants to go with and be ready for an extremely difficult knockout stage (England are on the worst side of the draw
).