A new rule reportedly being implemented.
Since the initial announcement of WWE bringing in wrestlers from the United Kingdom to their roster in 2017 the talents have been competing on special contracts. Those contracts allowed for the U.K. wrestlers to work for WWE and also take bookings with other promotions and companies with the exception of a few. The current WWE United Kingdom Champion Pete Dunne, Tyler Bate, Joseph Conners, Wolfgang, the recently signed Toni Storm and others were able to work their independent dates while still being contracted to WWE.
The competitors' contracts which allows them to do that has been altered a bit as it was announced by the British wrestling promotion 'Southside Wrestling' that they had to change their SWE Tag Team Championship main event match which featured NXT U.K. competitors Joseph Conners and Dan Maloney because the WWE's U.K. competitors are no longer allowed to work shows seven days out from any scheduled NXT U.K. live events or shows.
Dave Meltzer of F4WOnline followed up on this report noting that it is true that the WWE has established this rule. This is also coming off the helms of former PROGRESS World Champion Travis Banks dislocating his shoulder during PROGRESS' 'Coast To Coast' tour which was one day before he was scheduled to compete at the NXT U.K. TV tapings.