Coffey would regularly appear on the NXT UK brand, being accompanied by his brother Joe Coffey in a winning effort on the first ever televised match on NXT UK. They would both go on to form the stable Gallus alongside Wolfgang and feud with British Strong Style. At NXT UK TakeOver: Cardiff, Coffey and Wolfgang were unsuccessful in their plight to win the NXT UK Tag Team Championships from James Drake and Zack Gibson in a triple threat tag match, with the team of Mark Andrews and Flash Morgan Webster winning the match and the titles. Gallus would lose the titles to Pretty Deadly (Sam Stoker and Lewis Howley) on the February 25, 2021 edition of NXT UK, ending their reign at 497 days, the longest reign in the brand's history.
On the July 14, 2022 episode of NXT UK, Mark Coffey defeated Noam Dar to become the new NXT UK Heritage Cup Champion, but would soon lose it back to Noam Dar on the July 7 tapings.
At NXT Heatwave, Gallus made their NXT 2.0 debut, attacking Diamond Mine. On August 23, Gallus made their tag team debut on NXT, facing NXT UK Tag Team Champions Brooks Jensen and Josh Briggs, which they would go on to win via countout. At Worlds Collide, Gallus became the second team eliminated from the fatal four-way tag team elimination match for the NXT and NXT UK Tag Team Championship. Gallus would go on to have a rivalry with Briggs and Jensen, with the latter winning a no disqualification tag team match between the teams. Gallus were later suspended in September for attacking multiple NXT officials.
At New Year’s Evil on January 10, 2023, Gallus returned from their suspension and won a gauntlet match to become the number one contenders for the NXT Tag Team Championship, which they won at NXT Vengeance Day the following month.