Unless mortars and AT guns are only for the early game then the build order should be unaffected unless the player had no idea they were facing the UKF and made no effort to build mortars?
Most players don't lead with a build to counter emplacements because they are so out of meta. Generally emplacements go up by a certain point in the game as a cancer brit generally decides long before they want to play sim city but coh is already running. meaning delaying past that the enemy assumes they are not coming (which is usually the case) the counters are the same but they may not have been built. If the enemy has written off the possibility of them coming the shock can be effective in throwing them off balance and at the absolute very least they need to dedicate resources to taking it out (say you built it on a VP whik you can focus your efforts elsewhere when he is weakened. Idk why people only think there is ever one single solitary way to use anything