(deep breath)
Okay, so let's talk a little bit about large-caliber artillery, first of all.
Battleship rifles are:
- Expensive. Great Britain had (as seen above) no mounts sitting around at that point, and building new ones would have been terribly expensive.
- Manpower-intensive.
- A pretty major explosive threat. Magazines are armored for a reason!
- Not great for sustained operations. Total barrel life for the 14"/45 was rated at 340 rounds. Changing the barrel involved disassembling the gunhouse. Likewise, ammo capacity is intrinsically very limited (100 rounds per gun on KGV), and resupply is much harder than for medium-caliber weapons.
- High ship-impact. They produce lots of blast. They need a clear field of fire. They must be positioned carefully due to their weight and size. Their magazines are quite voluminous.
A good amphibious ship is:
- Cheap. Cheap cheap cheap. They aren't subjected to great shock or punishment, and have well-distributed loads, so can be built lightly.
- Full of troops and equipment. The payload fraction should be maximized to the greatest extent possible.
- Able to disgorge those troops and equipment quickly and efficiently. This means maximum-sized and reasonably uncluttered well decks and flight decks, to support high-tempo operations.
- I'll mention this one a second time: cheap. Cheap. Very very cheap.
Put these lists together and we see some major problems with the basic idea of a big-gun 'phib.
- You won't build a lightweight or cheap 'phib with a heavy-caliber gun turret. Just won't work. Battleships were heavily constructed for a reason - that huge concentrated weight is a real bugger to deal with. The shock from firing doesn't help either.
- If you go ahead and build it anyway, you'll find that such a LSD/LHD has pretty poor payload fraction compared to a traditional amphibious assault ship. That gun and magazine take up a huge amount of space and weight. The crew demanded by the system will impose other design constraints.
- Probably for the best that you can't carry much payload, because you won't be able to get it offboard as quickly. Your flight deck will end up small and cluttered (and a deck park will get wrecked by lots of gun ops), and the below-decks arrangement will suffer as well (which means slower loading of a smaller well deck).
- Self-defense will be an issue in the modern day. Blast and electronics don't mix very well.
There are some other intrinsic problems here.
- You've got, to be charitable, maybe 400 rounds of 14" ammunition aboard. Best-case, you've got full tube-life magazines, a total of 680 rounds. This is just barely more than half as many rounds as Spruance carries of 5/54 ammunition. Of course Spruance can do easy 5/54 UNREP, and probably replace barrels at sea. You can't. Reload means regunning, which means you're in port. Your entire mission is limited to maybe 400-600 rounds of gunfire support. Each round is very potent, but a 14" round can miss just as effectively as a 5/54. And for suppression, the difference is not a huge one.
- Even when both calibers were available, 5" ammunition was vastly more popular for NGFS in WW2. Battleships were quite often used as huge buckets of 5/38 ammunition, not to use their heavy guns. See Table 9 of Okinawa ops. Something like 4% of the total rounds expended were larger than 8". Something like 85% were 5/38 shells.
- Note star shell in that table above. Good luck convincing the MoD to buy either in 14" (same for smoke).
- You aren't going to have a lot of these around to make up for the gun scarcity problem, because they're expensive.
- NGFS is badly-conducted from the landing ship. In WW2, the USN found it helpful to conduct NGFS from the flanks of a landed formation. This is because range error is always much much larger than deflection error. You want to shoot across the length of the enemy line, not over the heads of your troops. Thus guns are best located some distance away from the landing ships.
Saying "support the troops they were landing" sounds very nice as a single sentence on paper, but in practice it would work very very badly unless you have almost arbitrarily large ships to play with. Note the USN looked at several large-caliber NGFS weapons during and after Vietnam (up to 12"), but these were always mixed with lots of 5/54 and were always separate ships, not built into the 'phibs. The 'commando conversions' of the Iowas were more to take advantage of a useful and existing hull, not as a sensible new-built configuration.