England lost 28–26, but Kallum helped the young Lions restore pride a week later with a 38–22 win at Wakefield Trinity Wildcats' Belle Vue.

He made 13 appearances in all scoring 4 tries, including a spectacular length of the field interception try late on to beat Huddersfield at Headingley 20–12.

[10] Later that year he played at centre for Leeds in the 2011 Challenge Cup Final defeat by the Wigan Warriors at Wembley Stadium.

[11][12] Watkins was given a boost after 2 injury-ravaged seasons when he was handed the Number 3 Jersey by the Leeds Head Coach Brian McDermott.

Kallum also signed a new 5-year contract on 26 January, keeping him at Leeds until 2017 at the earliest[13] He played in the 2012 Challenge Cup Final defeat by the Warrington Wolves at Wembley Stadium.

Watkins made his debut for the Gold Coast against Brisbane in Round 19 of the 2019 NRL season which ended in a 34–12 loss at Cbus Super Stadium.

[28][29] Watkins made a total of 6 appearances in his first season in the NRL as the Gold Coast finished last on the table and claimed the wooden spoon.

[30] In April 2020, it was announced that Watkins had quit the NRL to fly home back to England to be with his sick father.

[33] In round 21 of the 2022 Betfred Super League season, Watkins scored two tries for Salford in a 44-12 upset victory over St Helens RFC.

[34] In round 24 of the 2022 season, Watkins scored two tries for Salford in an upset victory over Catalans at the Stade Gilbert Brutus.

[35] In the 2023 Super League season, Sio played 23 matches for Salford and as the club finished 7th on the table and missed the playoffs.

He played in all five of England's matches at the tournament including the semi-final loss to Samoa where he made a number of defensive errors which lead to Samoan tries.