Currently, the longest successful engagement in wartime is at 3540 metres in Iraq in May 2017 by a Canadian (JTF-2). Canadian service members also hold records for three of the top five longest confirmed hits. Of note, two of these records have held up for 18 years. I believe the Brit you are referring to is at 2475 metres, the third longest record.
That said, if technology leads us to sniper accuracy out to 6.5 kms (roughly 4 miles), so much the better. But, with Pete’s comment firmly in mind, I wouldn’t be surprised if the platform for extended long range engagement comes through further development of drones as a platform.