Mk7 is the newer helmet introduced in mid 00s (2006 ish) to replace the older Mk6 which came in during the mid to late 80s. Updated design and protection.
for general purpose operational / combat head protection it is the ‘standard’ helmet, which is currently the mk7
for specific roles there may be a different helmet, such as para helmet, AFV/tank crew helmet, riot helmets, ATO/ bomb disposal specialist helmet etc
circumstances could dictate other use, such as on original Afghanistan deployments when the para helmet was withdrawn and they used the standard helmet instead
(the ‘best’ helmet can vary such as the need for a different style of helmet so you don’t get an injury when jumping due to the shape of a standard helmet, but sacrifice a point of protection on the ground. If you’re not parachuting then that compromise might not be worth it)
Other ‘protection’ could be from the cold and using an old school chopped up jumper sleeve or a ‘cap comforter’ (commando hat), or even a beret or field cap are a degree of head protection with at least basic head insulation