No, it isn’t. The only legal security requirements are set out in Conditions 4 and 4(a) on your firearm certificate, which simply require you take reasonable precautions to deny access to firearms and ammunition to an unauthorised person.
Anything else you hear, whether from the Home Office, from the Police, from me or from any other self-appointed ‘expert’, is just advice or guidance, not a legal requirement.
In deciding what is reasonable in any particular circumstances we might bear in mind the Home Office’s statement that rifles are not generally seen as being attractive to criminals.
Having said that, received wisdom is that we should lock up ammunition separately and since firearms cabinets usually have a compartment specifically for that purpose, you might as well use it.
However, I am always a bit wary of removing the bolt from a rifle.
Leave it open (the bolt, not the cabinet) to allow free circulation of air but at least if the bolt is attached to the rifle you are less likely to find yourself out on your stalking ground with a rifle and ammo but no bolt.
I’ve seen that happen more than once and it’s one of the reasons I’m such an advocate of single shot rifles.