The regiment portrayed throughout this series is an armoured regiment. That is plain from the WWI tank on the cloth embroidered cap badge. But a real regiment, set in this modern, time frame, would have the Queen's crown surmounting the laurel wreath encircling the tank. This would not be permitted, so a nondescript design was used in its stead.
For this, and all following installments in which the military is shown, the members of the armoured regiment should be wearing black berets, not khaki. From the beginning of the tank era, in WWI, armoured regiment soldiers have worn the black beret. Other regiments have different colours.
Derek Riddell (Adam Gettrick) & Daniel Ezra (Daniel Reed) also worked together on Undercover (2016) as Paul Brightman & Dan Johnson respectively.