So you think it is ok to let some novice FAC holder go out and get a .50 cal or similar and then go out and shoot small deer with it?

I think you may have just summed up exactly why firearm justification restrictions are required!
MS
Um, pretty much yeah. I am certainly no expert, but surely the bloke is either safe with a rifle or he isn't. A backstop that is unquestionably safe for a .270 will also be safe for a 375 or a 458 lott for that matter.
Obviously there are reasons why a DG chambering isn't ideal for a muntjac; recoil, noise, ammo cost, rifle weight, the struggle to find a range to zero it at, not to mention the possibly excessive meat damage, but as far as safety and deer welfare (which should be as far as the polices remit goes) is concerned, then I see no issue. Trying to argue that a certain chambering is more dangerous purely because it is more powerful is silly when you get to deer legal calibres, the bloke behind the rifle is the only factor for that. People mock the system endlessly for promoting the myth that a .243 is magically safer for a first grant than .308 or whatever, so how is this any different?
I present to you the following:
You are sat in a highseat waiting for a Roe deer. One wanders into view at around 60yds.
a.) You check and the shot is good. You line up and fire your .223 (in Scotland obviously). The deer drops and the bullet ploughs into the ground 10yds behind, kicking up some small amount of debris. Excellent, steak for dinner!
b.) You check and the shot is good. You line up and fire your .577 tyrannosaur. The deer drops and the bullet ploughs into the ground 10 yds behind, kicking up 10 kilos of dirt. You massage your shoulder and moan about the cost of powder. Ah well, at least you won't need to get the mincing machine out.
Is a safer than b?
or in this situation
You have arrived at the end of a long hill stalk on reds. You see your quarry at the bottom of a gully around 150yds away.
a.) You check and the backstop looks good. You line up your .270, sort your bullet drop and fire. the deer walks a few steps and collapses. The bullet impacts the safe backstop. The stalker smiles
b.)You check and the backstop looks good. You line up your .500NE, sort your bullet drop and fire. The deer drops like a stone. The bullet impacts the safe backstop, forming an impressive hole. The stalker scowls, wiggling his finger in his ear and muttering about small penis syndrome.
Alternatively;
You are stalking in a forestry block and spot a fallow deer on the skyline. You look hard and decide that the ground behind drops of sharply and your only backstop is a reasonably substantial tree.
a.) You think hard, but ultimately decide it isn't worth the risk. You slip the front lens cover back into place on your .243 and try and get into a better position.
b.) F*ck no, the .375 H&H never even leaves your shoulder. There is always another day...
Can you list any situation in the field where a shot would be safe with a .243 where it wouldn't be with even a .500NE? If anyone can I would be genuinely interested to hear it, because I can't.
The only possible safety justification I can think of is that if the worst were to happen, you would have more chance of surviving being shot with a smaller bore, but I ask you, if the police think this is a genuine risk for an applicant, why are they granting him anything at all?