In what proved to be a markedly good first professional season at Worcester, Singleton made twenty appearances.
[3] On 11 March 2020, Singleton joined Gloucester on a season-long loan deal for the 2020–21 season after Saracens were relegated to the RFU Championship due to salary cap breaches.
[10] In May 2017 Singleton received his first call-up to the senior England squad by coach Eddie Jones and started in a non-cap game against the Barbarians.
[15] On 11 August 2019, Singleton won his first cap for England off the bench against Wales at Twickenham in a warm-up match and the next day his selection for the 2019 Rugby World Cup was confirmed.
[3][18] Singleton was called up to the England squad on 15 November 2021 as cover for the injured Jamie George for the game against South Africa during the 2021 Autumn Nations Series.