There are probably twelve certainties already:
Cook, Stoneman, Root, Malan
Bairstow
Stokes, Ali, Woakes
Anderson, Broad, Roland-Jones, Wood
So, they need one more batsman for the Test XI, a reserve, a spare keeper, and then whatever spare spinners and seamers they want to take.
I think the keeper will be Foakes, and Bayliss has more or less said Hales will be one of the batsmen. If there is a second spinner, it'll be Crane, because Rashid will not be a positive influence carrying drinks for three months, and another seamer would be a toss up between Ball and Overton (tall, horses for courses pick).
The last batting slot is interesting. Westley might have dragged it out of the fire, but they will also talk about Hameed, Vince and Ballance.