Asa Butterfield as Nathan Ellis

Rafe Spall as Martin Humphreys

Sally Hawkins as Julie Ellis

Eddie Marsan as Richard

Jo Yang as Zhang Mei

Alex Lawther as Isaac Cooper

Alexa Davies as Rebecca Dunn

Martin McCann as Michael Ellis

Orion Lee as Deng Laoshi

Jake Davies as Luke Shelton

Paul J. Dove as John

Percelle Ascott as Ben Morgan

Christina Low as IMO Contestant

Edward Baker-Close as Nathan Ellis (9 yrs)

Suraj Rattu as Pav Kamdar

Jamie Ballard as Headmaster

Clare Burt as Doctor

Adam Foster as Paediatrician

Lee Zhuo Zhao as Li Jing

Shannon Beer as Caz

Tasha Connor as Jessie

Bo-Han Huang as Luke's Buddy

Song-Hung Chang as Employee

Lawrence Jeffries as Schoolboy

Ciaran Wakefield as Schoolboy