Perera is the most experienced but Shanaka has probably done the best (albeit on helpful tracks). In my opinion, Shanaka is a better "all rounder" but they will probably pick Thisara.
Thisara isn't a seam bowler anymore. He just bowls 1 seam-up along with 5 slower balls in an over. But he should be in the team just for batting, he's certainly improved his batting. Dasun's bowling was threatening vs England 1st Test and the Ireland ODI's. So Dasun should also earn a place. Having said that, the weather in England is already looking sunny and dry, so the ball might not swing around that much next month.
Both Shanaka and Thisara should play.
Chamika Karunarane also looks impressive
4. Kusal Mendis
9. Amila Aponso
10.Nuwan Pradeep/ Lakmal
Shanaka for sure.. Thisara isn't a frontline allrounder anymore.. Shanaka definitely deserves more opportunity.. He wasn't selected consistently..
The reality is we should probably play both but given the selectors dislike for Shanaka and Thiseras back to back fifties, it will be Tisera.