Well, I'm going to bring up two points regarding AA for UK. The gunners, and the guns.
Firstly, the gunners. UK, as one of the original 4 nations, has dedicated AA gunner classes of the old design. Promoting to AA classes increases AAW ability, but reduces other abilities. However, where this was once of benefit a very long time ago, this is no longer the case. AAW ability no longer influences AA gunner performance (or anything at all). AA gunners rely on the attributes of normal gunners, Reload and Accuracy, and as I mentioned before these get lowered when you promote to the AA gunner class.
But you want the AA gunner class for the AA special guns, so what's the solution? Well, classing UK AA gunners and trying to make do is one method, but being able to reach that all important reload ability cap can be difficult and requires a combination of high base reload ability and a lot of experts and vets. A more efficient alternative is to keep your gunners at the basic Armament Sailor class (known as delay classing) to give their Reload and Accuracy more time to develop, then classing them into AA gunner at higher levels. You will still be able to AA with normal guns (because the specialised AA guns, often refered to as A guns due to the A suffix, are only AA variants of normal guns) until you access the A guns. The third option is to level a pair of normal gunners and class them like normal gunners, with high base reload and reload promotions. This option makes it the easiest to cap your gunners reload ability, but you won't be able to access the A type guns, you will need to use the slightly less effective D versions. You also won't have access to the UKs unique AA gun, the 2dpr 'Pom poms'.
Delaying your gunners is highly recommended; boosted elite reload gunners that have been classed on time still need to be lvl 120 and have 250 vets to reach the reload cap. Relatively, if you kept that same boosted gunner as an armament sailor then with just 100 vets they will reload cap at lvl 105, and with 200 vets this drops to only lvl 88. If you wanted gunners that are reload capped at 120, but wanted to access AA guns ASAP, delaying the above boosted elite reload gunners to lvl 70 will allow you to class them into AA gunner to access the AA guns and still reach the reload cap at lvl 120 with only 100 vets. (Note, these example all consider sailors having full experts)
These are only examples of the advantages of delaying your gunners as armament sailors. If you do so, I recommend you keep an eye on your gunner reload ability to see how close they are to the reload cap so you know when you can class into the actual AA class. You can delay to get access to guns earlier without need excessive amounts of vets, you can delay to make your gunners lighter, you can delay to reach the reload cap ASAP. Delaying the promotion to AA classes is a wise decision and investigating what levels your own gunners will cap with a level of vets you are comfortable with is highly recommended.
Now, the guns themselves. I'll point out the three most common and their angles.
The first, lower level AA guns do medium damage and have the fastest reload of the UK AA guns at 2.2s (only just) as well as being incredibly light at 35t each. The main drawback of this gun is its incredibly short range. I only recommend using this gun if you cannot access the 2x 5.25" RP 10s below.
Level 16 2x 4"/45 cal QF HK Mk. XVI D (The normal class gun) Golden Angle 39 and reload of 2.32s
Level 31 2x 4"/45 cal QF HK Mk. XVI A (The AA class gun) Golden Angle 40 and reload of 2.2s
The second gun is the most effective conventional UK AA gun, it has medium range, almost twice the damage of the 2x 4" guns, and only slightly slower reload speed at 2.24s. As an AA gun it performs only moderately in every way, no real drawback, but no distinction either (though they do turn quite quickly). Still, it is probably the best the UK has. It weighs a rather heavy 115t a piece however.
Level 31 2x 5.25"/50 RP 10 Mark I L has a Golden Angle of 32 and reload of 2.72s
Level 48 2x 5.25"/50 RP 10 Mark I A has a Golden Angle of 36 and reload of 2.24s
2x 5.25"/50 RP 10 Mark I A
The third and final gun I will mention is the UKs unique AA gun, the short range, rapid fire "Pom poms".
Level 45 7x 2pdr 1.57"/39 Mark VIII has a Golden Angle of 45-46 and reload of 0.52s
It is difficult to compare this gun to others, on paper its damage is very low, but in reality it can apply a decent amount of damage very quickly. Its main flaws are it's short range and awkward angles, it is quite ineffective at shooting down enemy scouts. Weighing in at 96t is fairly average.
7x 2pdr 1.57"/39 Mark VIII
I hope some of this helps, if you need more detailed information regarding late classing gunners then let me know, I'll help you figure out how many vets/experts your gunners might need or when to promote them from armamemt sailors if you choose to reroll a pair of gunners.