Couple of things here on England:
1. England's MF was always the weak point even coming into the tournament. As an LFC fan I have no issue taking the shit out of our competitors players but in this case I think some slack should be cut for Alli though Lingard has an entirely punchable face so I have nothing good to say about him regardless. Either way neither play at club level as midfielders so it's not surprising they were out of their depth, Ox's injury was a big blow to the national team as he was just what they needed. Someone who could progress the ball through midfield on a dribble. They didn't have anyone that could do that.
2. The solution to #1 was then to make Kane drop really deep to facilitate for Sterling, Alli and Lingard. He's good at scoring goals, not as much at creating and in the end I think it backfired as Kane was always really far away from goal.
3. England's youth teams have won almost everything there is to win in the last 18-24 months, they have some really special players on the way up so I wouldn't look at this as a last chance. Biggest issue is finding EPL teams that will actually give most of them a chance to play and why Jadon Sancho went to Dortmund. If they can figure out a way to let the next wave grow then they should have players that can hang with the best in the world for the next 10 years. Sancho, Foden, Brewster all look like potential superstars.
As far as International vs. Club soccer, International soccer can never reach the same level of club soccer for a variety of reasons. Ossoi is right that it's not a team that's thrown together once every 4 years but at the same time they spend no where near the amount of time together as they do at club level plus you can't exactly transfer in players to cover for weaknesses. Combined with it being easier to coach a good defense vs. a good attack and I think it's always somewhat been this way when it comes to international tournaments.
Do the top 4 even field 50% of britishs ?
Not exactly 50% but it's close due to UCL rules on homegrown players.
Edit: Actually yes based on the current Top 4.
ManC - 4 players
ManU - 4 players
Spurs - 5 players
LFC - 2 players
That's 15/23 players in the Top 4 clubs.