A lot of the ex pro's/managers/pundits always talk about how it is good to have club partnerships in an international team. The synergy will already be there as it has built up over the course of a year and even a month's worth of training sessions + one or two friendlies can't replace that. Some international teams are slow to get going whilst they find their feet together, in the group stages we will need every advantage we can get and having a team that are already on the same wavelength is a big plus.
As for Barkley, I think he's a great player but as one of the deeper two in that three man midfield he would be wasted. For me he is the furthest forward of the midfielders, which means either switching out Lallana, Sterling or Rooney.
There is obviously something to be said for the first point, but there has to be a limit surely?
When you look at what someone like Henderson really offers to the Liverpool team, it becomes very much clear that he is replaceable and as such not integral to England's chances at the World Cup. His near ever-presence next to Gerrard is not a wholly good thing - it is also a sign of Gerrard's limitations and how they were managed by Rodgers with the players he had at his disposal. Gerrard has never had supreme control or agility in tight spaces (though to exclude Gerrard from the category of 'creative midfielder' on this basis is diabolical); this, coupled with his declining pace, render the inclusion of a 'workhorse' like Henderson vital. A willing runner, with defensive qualities, who is fluent in a pressing game that can afford Gerrard the extra seconds he may need to perfect a 60 yard pass, is what is required and is evident in Henderson. But realistically, Henderson is a weak link in the team and is made important only by the paucity of players with his qualities in the Liverpool squad.
Hodgson, on the other hand, has a much better selection of players to pick from that can quite easily perform Henderson's limited role and offer more in the same/other areas. Milner and Barkley are these players. In Milner especially we are talking about a seasoned professional who has won two titles and captained England, playing with some of the best players in the world week-in, week-out, while Henderson was one of the central foci for ridicule of Liverpool until this season, classed in the same league as Downing and Carroll - he has only been elevated from these depths because his limited ability has been maximally utilised in the Liverpool team. Milner is often explicitly used in the 'workhorse' role for City and easily matches Henderson in all of his tangible qualities. In terms of the intangibles - Milner has vast experience compared to Henderson and while he cannot match the latter's partnership with Gerrard, will have played with him for England. In all I think Milner offers far more than Henderson, and the fact that Gerrard and Henderson are club-mates should not outweigh the other factors cited above over the course of the tournament. In the first game? Maybe, but even then, his and Milner's defining qualities may be less relevant against Italy, with Gerrard getting more time on the ball and someone a little more technical and perhaps less physical required, such as Wheelchair or even Barkley.
Thereafter, if a player like Henderson is required, Milner is the obvious choice.
Regarding the second point: Barkley is not creative enough to be the most advanced central midfielder. England's main problem in putting together a decent international team recently is a lack of creativity, someone who can unpick a defense where it seems there is no space. Barkley is just not this player, and to take out either of Rooney or Lallana takes out the two players most capable of playing these sorts of balls. And for all you Arsenal apologists - Wheelchair is not this player either, very rarely does he split a defense.
In terms of potential box-to-box midfielders available to England, Wheelchair is undoubtedly the most capable passer of him, Barkley, Milner and Henderson, and is capable on the ball more generally - he will have his use in games where space is tight and additional box-to-box creativity from is needed to assist Gerrard and Lallana - but he does not offer quite the same threat as Barkley's power. Playing Barkley at #10 would detract from his ability to use this power, which most readily manifests itself (coupled with his technical ability) in his on and off the ball running.
England have more suitable candidates to play in the hole than Barkley, and he is more suited to a slightly deeper role anyway. Again, I think he would be suited to the 'workhorse' role next to Gerrard, and offers more than Henderson by virtue of his exceptional ability to drive with the ball.
I realise that in reality, Milner will be used on the right as a massive drain on the team's creativity, while Henderson will further drain this creativity when played in the centre. Lallana will probably start on the bench, and as always Rooney and Gerrard will not live up to the thankless task of providing for the whole England team. But please, Hodgson, if you're reading this, Gerrard, Lallana and Rooney are the most important midfielders; complement them with Milner, Barkley and Wheelchair as you see fit, but do not pick Henderson!!
Ah what do I care, it's only England.