10 of the fifteen are absolutely nailed on as
@Manormanic says - Roy, Bairstow, Root, Morgan, Buttler, Stokes, Moeen, Woakes, Rashid from the starting XI plus Hales as backup for the top order.
So... that leaves 2 spots for starting seamers, probably 2 backup seamers and a backup allrounder who preferably bowls spin. For the seamers, it's obviously four from Willey, Plunkett, Wood, Curran and Archer. Plunkett is in for me as the only middle overs specialist, unless he loses form badly. Willey is lefty, experienced and an excellent allrounder so he's in. Probably leave Wood out as the most one-dimensional bowler from the last three. Denly's currently in the hot seat for the last spot, Dawson is probably the closest competition? But... in the hypothetical situation where you need injury cover for Moeen in a big game...? I'd be strongly tempted by Samit.